Cathodes and electrolytes for rechargeable magnesium batteries and methods of manufacture

ABSTRACT

The invention relates to Chevrel-phase materials and methods of preparing these materials utilizing a precursor approach. The Chevrel-phase materials are useful in assembling electrodes, e.g., cathodes, for use in electrochemical cells, such as rechargeable batteries. The Chevrel-phase materials have a general formula of Mo6Z8 (Z=sulfur) or Mo6Z18-yZ2y (Z1=sulfur; Z2=selenium), and partially cuprated Cu1Mo6S8 as well as partially de-cuprated Cu1-xMgxMo6S8 and the precursors have a general formula of MxMo6Z8 or MxMo6Z18-yZ2y, M=Cu. The cathode containing the Chevrel-phase material in accordance with the invention can be combined with a magnesium-containing anode and an electrolyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuation-in-part patent application claims priority fromdivisional U.S. patent application Ser. No. 15/897,721, filed Feb. 15,2018, which claims priority from U.S. patent application Ser. No.14/325,891, filed Jul. 8, 2014, which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/843,647, filedJul. 8, 2013, entitled “Cathodes and Electrolytes for RechargeableMagnesium Batteries and Methods of Manufacture”, which are hereinincorporated by reference.

GOVERNMENT FUNDING

This invention was made with government support under grant#DE-FE0004000 awarded by the Department of Energy-NETL. The governmenthas certain rights in the invention.

1. FIELD OF THE INVENTION

The invention relates to Chevrel-phase materials and methods for theirpreparation, and to secondary electrochemical cells utilizing amagnesium-containing anode and a Chevrel-phase-containing cathode.

2. BACKGROUND

High energy density, rechargeable electrochemical cells are known in theart. A rechargeable cell is theoretically capable of charging anddischarging indefinitely. In producing a rechargeable battery system, amaterial for the cathode is selected. In certain instances, the cathodematerial is in the form of a liquid which allows reactions to readilytake place. However, when in the form of a liquid, provisions are madeto keep the cathode active material away from the anode, otherwiseself-discharge can occur. As an alternative, the cathode material can bein the form of a solid which is essentially insoluble in theelectrolyte. The solid cathode material is selected such that it absorbsand desorbs the anode ion because solubility of the anode ion occursreversibly during operation of the cell. Such a solid cathode can becapable of intercalation of ions which are solubilized by theelectrolyte. The electrolyte is selected to permit electroplating ofsolubilized ions at the anode. The plating of ions at the anode occursduring recharge of the cell and the intercalation of the cathode occursduring discharge of the cell.

Chevrel-phase compounds (CPs), also referred to as Chevrel materials,include an invariant portion which may consist essentially of molybdenumand a chalcogen. The chalcogen can be selected from elements in Group 16of the Periodic Table, including sulfur, selenium, tellurium or mixturesof these, with or without minor amounts of oxygen. Ordinarily, thisfixed portion has a stoichiometric formula of Mo₆Z_(y) where Zrepresents the chalcogen and y is usually between about 7.5 and 8.5,most typically about 8. The unique crystal structure of the materialspermits intercalation of metals, so that the overall stoichiometry ofthe Chevrel-phase material is represented as M_(x)Mo₆Z_(y) where Mrepresents the intercalated metal and ‘x’ may vary from 0 (nointercalated metal) to an upper limit which may be about 4 or lessdepending upon the particular metal.

Ternary CPs are a unique class of cluster compounds which exhibitremarkable magnetic, thermoelectric, catalytic, and superconductiveproperties. The crystal structure of CPs consists of Mo₆-octahedronclusters surrounded by eight chalcogen (e.g., S and/or Se) atoms at thecorners of a distorted cube. For example, Mo₆S₈ units are linked witheach other and form a three-dimensional framework with opencavities/channels that can be filled with wide-variety of guest atomsand form ternary CPs M_(x)Mo₆S₈ (0<x<4). However, Mo₆S₈ binary CPscannot be synthesized directly and indirectly stabilized via leachingmetal from their ternary counterparts. For example, Mo₆Se₈ binary CPsinclude an iso-structure wherein Mo₆Se₈ clusters are rotatedapproximately 26° about the body diagonal (3 axis) of the rhombohedralsymmetry (R3) which allows for bonding of Se atoms of one cluster to aMo atom of a neighboring unit. The resultant three-dimensional Mo₆Se₈framework has open cavities/sites that can be filled completely in theM_(x)Mo₆Se₈ CPs into triclinic (P1) forms due to intrinsic latticeinstabilities. Among the three different families of CPs (Mo₆Z₈, Z═S,Se, Te), sulfide CPs have high ionic mobility at room temperature whichallows them to transport monovalent (Li⁺, Na⁺) and bivalent (Mg²⁺)cations, and to act as a cathode for rechargeable batteries.

Energy is released upon intercalation of the metal into the CPs and asthe intercalation process is partially or wholly reversible with certainmetals, the CPs can therefore be used as cathodes in electrochemicalcells.

A cell with a lithium anode and a Chevrel-phase cathode of the formulaLi_(x)Mo₆S₈, for example, can be subjected to a charge cycle in whichlithium is removed from the Chevrel-phase by the applied electricalenergy. In a discharge cycle, the lithium is re-intercalated into theChevrel-phase releasing energy as electrical energy. The reactionmixture containing lithium, molybdenum and sulfur for direct formationof the lithium-intercalated CPs can be prepared by heating a precursormixture. The precursor mixture including a heat-liable lithium compoundtogether with molybdenum and sulfur, typically as a mixture of MoS₂ andfree Mo. Upon heating, the heat-labile compound yields volatiledecomposition products which may be swept from the mixture, e.g., by astream of inert gas, leaving behind the lithium, molybdenum and sulfurto form the Chevrel-phase material.

Intercalation reactions in typical battery development have focused onthe use of alkali metals, specifically lithium as anodes. In comparison,there has been less research with respect to the use of alkaline earthmetals, such as magnesium, for use as anodes and the use of cathodescapable of intercalation of alkaline earth metal ions.

It is known in the art to use lithium ion batteries for a wide varietyof energy storage applications due to their very high energy density andflexible design. In considering alternative materials to lithium inproducing electrochemical batteries, it is acknowledged thatmagnesium-based energy storage systems may be considered suitablealternatives because magnesium is environmentally safe, cost effectiveand abundant in the earth's crest. Further, magnesium is bivalent andtheoretically capable of rendering higher volumetric capacity thanlithium.

It has been found that conventional salts such as Mg(ClO₄)₂,Mg(CF₃SO₃)₂, Mg[(CF₃SO₂)₂N]₂ and the like, dissolved in variousnon-aqueous solvents develop surface passivation on a magnesium anodeand effectively block Mg²⁺ transport. Relatively fast and easyintercalation of Mg²⁺ ions at room temperature makes CPs a preferredmaterial of the cathode for magnesium batteries. However, synthesis ofthe thermodynamically unstable Mo₆S₈ phase is challenging. Typically, aCuCP (Cu_(x)Mo₆S₈) is synthesized by solid state reactions of elementalblends of copper, molybdenum, and MoS₂ powders in an evacuated quartzampoules at a temperature of approximately 1150° C. for one week or by amolten salt approach including heat treatment at approximately 850° C.for about 60 hours under an argon atmosphere. Both of these approachesrequire chemical leaching in solution for several days at roomtemperature for complete removal of the copper.

The conventional methods utilized are not convenient approaches that canbe readily and effectively translated into large scale manufacturingprocesses for generating the Chevrel phase. For example, an elementalblend sealed in evacuated quartz ampoule and heated at high temperature(˜1273 K) for a long duration (˜7 days) to obtain Cu_(x)Mo₆S₈ results inhigh manufacturing costs not withstanding generating reproducible largescale quantities. Further, a significant disadvantage is the high vaporpressure of sulfur inside the ampoule during heating which causes asafety hazard. Furthermore, the final product obtained from the quartzampoule has non-stoichiometry due to sulfur volatilization andcondensation on the ampoule walls further from the product and excesssulfur that may be hence required to compensate for sulfur vapor lossduring heating. Attempts to use metal sulfide (CuS, MoS₂) instead ofelemental sulfur to avoid the high vapor pressure, has resulted in longand arduous synthesis time which is unreasonably long and thus notamenable large-scale production. Moreover, it was found that there canbe difficulty in forming a reaction product at high temperature, e.g.,the Chevrel phase may not be obtained and instead a sulfur deficientphase can form which adversely affects the desired and expectedelectrochemical performance.

In an attempt to reduce total synthesis time duration involved duringthe solid-state reaction method, cold-pressing and hot pressing havebeen employed as alternative means for the synthesis of a Cu_(x)Mo₆S₈phase. Neither the milling time nor the sintering temperature has beenoptimized for the synthesis of Cu_(x)Mo₆S₈. However, it was found thatpressure-assisted sintering at elevated temperature ˜1123-1473 K wascapable of reducing the synthesis duration to ˜5-8 hours as compared to7 days required for solid-state synthesis in an evacuated quartzampoule.

The desired CPs also have been synthesized from soluble sulfideprecursor, e.g., polythiomolybdate and metal salt, which form a chelatedcomplex in ether or methanol solvent and directly forms the desiredCu₂Mo₆S₈ phase when heated at ˜1073-1273 K under hydrogen atmosphere. Analternative soluble precursor method was proposed for the synthesis ofnickel and lithium ternary CPs.

Obtaining Mo₆S₈ CPs from known soluble sulfide precursor methodsrequires the use of hydrogen gas during sulfurization. Final reductionof sulfur compounds to desired Chevrel phase at elevated temperatureneeds strict regulation and skills, and poses a safety concern for thesynthesis of CPs.

Thus, there is a need for improvements in electrochemical cellsincorporating Chevrel-phase cathodes and in the synthesis methods forthe CPs, and furthermore, in the use of alkaline earth metals, such asmagnesium, as anodes. Moreover, there is a need in the art to developtime-saving economical approaches and methods for the direct synthesisof CPs. In this respect, high energy mechanical milling (HEMM) isemployed as a scalable, economical time-saving approach for the directsynthesis of ternary metal CPs (M_(x)Mo₆Z₈ and M_(x)Mo₆Z¹ _(8-y)Z² _(y),wherein, for example, Z is S or Se, Z¹ is S and Z² is Se) using MZ, Moand MoZ₂ or MZ¹, MZ², Mo, MoZ₂ ¹ and MoZ₂ ² as the precursor. Theinvention provides easy, rapid and facile precursor approaches for thesynthesis of CPs for use as a cathode and magnesium-containing materialfor use as an anode in an electrochemical cell.

SUMMARY OF THE INVENTION

In one aspect, the invention provides an electrochemical cell includingan alkali metal-containing anode, a cathode and an electrolyte. Thecathode includes a Chevrel-phase material of a formula Mo₆Z¹ _(8-y)Z²_(y) derived from a precursor material of a formula M_(x)Mo₆Z¹ _(8-y)Z²_(y) wherein M is a metallic element, x is a number from greater than 0to 4, y is a number from greater than 0 to less than 8, and each of Z¹and Z² is a chalcogen with or without the presence of oxygen, wherein Z¹is a different chalcogen than Z².

In certain embodiments, the alkali-metal-containing anode comprisesmagnesium.

In certain embodiments, the metallic element M can be selected from Li,Na, Mg, Ca, Sc, Cr, Mn, Fe Co, Ni, Cu, Zn, Sr, Y, Pd, Ag, Cd, In, Sn,Ba, La, Pb, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu andmixtures thereof.

Each of the chalcogens Z¹ and Z² can be selected from chemical elementsin Group 16 of the Periodic Table, including sulfur, selenium, telluriumand mixtures thereof. Further, each of Z¹ and Z² can be sulfur,selenium, tellurium or a mixtures thereof, with or without the presenceof oxygen. Wherein, Z¹ is a different chalcogen than Z².

In certain embodiments, M is copper, x is 2, Z¹ is sulfur, and Z² isselenium.

The precursor material can be formed from a mixture of MZ¹, MZ², MoZ₂ ¹,MoZ₂ ² and molybdenum.

The Chevrel-phase material can be of the formula Mo₆S_(8-y)Se_(y) whichis derived from a precursor material of Cu₂Mo₆S_(8-y)Se_(y) and the saidprecursor material Cu₂Mo₆S_(8-y)Se_(y) which can be derived from amixture of ammonium tetrathiomolybdate and anhydrous CuCl₂ in thepresence of anhydrous N,N-dimethylformamide.

The electrolyte can be in the form of an electrolyte solution includingelectrolyte metal salt and the desired solvent. In certain embodiments,the electrolyte solution includes amidomagnesium-based magnesium salttransmetallated with an aluminum salt electrolyte dissolved in thesolvent. In other embodiments, the electrolyte solution includes phenylmagnesium chloride-aluminum chloride, amidomagnesium-based magnesiumsalt transmetallated with an aluminum salt electrolyte, and solvent. Thesolvent can be tetrahydrofuran.

In yet other embodiments, the electrolyte solution includes3-bis(trimethylsilyl)aminophenylmagnesium chloride with aluminumchloride in tetrahydrofuran.

The electrochemical cell can be a rechargeable battery.

In another aspect, the invention provides a method of synthesizing aChevrel-phase cathode material. The method includes preparing aprecursor material of a formula M_(x)Mo₆Z¹ _(8-y)Z² _(y), wherein M is ametallic element, x is a number from greater than 0 to 4, y is a numberfrom greater than 0 to less than 8, each of Z¹ and Z² is a chacogen withor without the presence of oxygen, wherein Z¹ is a different chalcogenthan Z², and removing the metallic element from the precursor materialto form a Chevrel-phase cathode material of a formula Mo₆Z¹ _(8-y)Z²_(y).

Each of Z¹ and Z² can be selected from the group consisting of sulfur,selenium, tellurium, wherein Z¹ is a different chalcogen than Z².

In certain embodiments, preparing the precursor material includescombining stoichiometric amounts of MZ¹, MZ², MoZ₂ ¹, MoZ₂ ² andmolybdenum. Furthermore, preparing the precursor material can includecombining stoichiometric amounts of ammonium tetrathiomolybdate,anhydrous copper chloride and N,N dimethylformamide to form the desiredmixture. Furthermore, preparing the precursor material can includeheating the mixture to at least substantially complete reaction,filtering, precipitating, drying the precipitate to yield a precursormaterial of a formula M₂Mo₆Z¹ _(8-y)Z² _(y), completely or partiallyremoving M ions, and drying to obtain the Chevrel-phase cathode materialof Mo₆Z¹ _(8-y)Z² _(y).

In other embodiments, preparing the precursor material includescombining stroichiometric amounts of copper (II) sulfide, copper (II)selenide, molybdenum, molybdenum disulfide and molybdenum selenide.Further, preparing the precursor material can include high energymechanical milling the mixture to form a powder, heating themechanically milled powder to at least substantially complete thereaction, yielding a precursor material of a formula M₂Mo₆Z¹ _(8-y)Z²_(y), completely or partially removing the M ions, and drying to obtainthe Chevrel-phase cathode material of a formula Mo₆Z¹ _(8-y)Z² _(y). Inanother aspect, the invention provides a method of preparing anelectrode.

In another aspect, the invention provides an electrode. The electrodeincludes a slurry and a current collector, wherein the slurry is atleast partially deposited onto the current collector to form a coatingthereon. The slurry includes a Chevrel-phase cathode material of aformula Mo₆Z¹ _(8-y)Z² _(y).

The current collector can be a material substrate selected from thegroup consisting of copper, graphite, nickel, platinum, quartz, gold,stainless steel, tantalum, titanium, silver and mixtures thereof. Incertain embodiments, the current collector is graphite foil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot that shows powder XRD pattern of Cu₂Mo₆S₈ (Cu₂ CP)(top) obtained by high energy mechanical milling of CuS, MoS₂, and Mocorresponding to the stoichiometric compositions followed by heattreatment at 1000° C./5 h under UHP Ar, and acid leached Cu₁Mo₆S₈ (Cu₁CP) (bottom) obtained after 2 days to partially leach copper using 6 MHCl solution, in accordance with certain embodiments of the invention;

FIG. 2a is a plot that shows cycling data, FIG. 2b is a plot that showsgalvanostatic discharge-charge profile, and FIG. 2c is a plot that showsrate capability of the synthesized Cu₁Mo₆S₈ electrode following leachingof Cu₂ CP when cycled against magnesium in Mg-ion battery, in accordancewith certain embodiments of the invention;

FIG. 3a is a plot that shows a galvanostatic discharge-charge profile ofCu₁ CP electrode at 1^(st), 2^(nd), and, 3^(rd) cycle carried out at ˜5mAg⁻¹ current rate within the potential window of 0.5-15 V, FIG. 3b is aplot that shows differential capacity, dQ/dV versus voltage (V) curvesof 2nd and 3rd cycle, FIG. 3c is a plot that shows ex-situ XRD analysisat 0.5V and 1.5V cut-off voltage during charge/discharge cycles, andFIG. 3d is a plot that shows cycling data of Cu₁ CP performed at a slowcurrent rate of 5 mAg⁻¹ within 0.5-1.5V potential window in a Mg-ioncell, in accordance with certain embodiments of the invention.

FIG. 4 is a plot that shows XRD patterns collected on the powdersobtained following mechanically milling for 1 h, 2 h, and 3 h usingstoichiometric amounts of the individual elements, sulfide and selenideprecursors corresponding to Cu₂Mo₆S₇Se₁ stoichiometry and also onpowders obtained after heat-treating at 1000° C. for 5 h showing theformation of Cu₂Mo₆S₇Se₁ and acid leached Mo₆S₇Se₁, respectively, inaccordance with certain embodiments of the invention;

FIG. 5a is a plot that shows a cyclic voltammogram (CV) curve obtainedwith a sweep rate of ˜50 μVs⁻¹ showing Mg-ion insertion/extractioninto/from the synthesized Mo₆S₇Se₁ electrode, FIG. 5b is a plot thatshows galvanostatic cycling data at a current rate of ˜20 mAg⁻¹, FIG. 5cis a plot that shows capacity versus voltage profiles, and FIG. 5d is aplot that shows differential capacity versus voltage (dQ/dV versus V)curves for the 2^(nd), 50^(th), and 200^(th) cycles, in accordance withcertain embodiments of the invention;

FIGS. 6a, 6b and 6c are plots that show XRD patterns collected on thepowders after mechanically milling for 1 h, 2 h, and 3 h correspondingto the stoichiometric ratio of Cu₂Mo₆S_(8-x)Se_(x) (x=3, 4, 5) and afterheat treating at 1000° C. for 5 h showing the formation of (a)Cu₂Mo₆S₅Se₃ and acid leached Mo₆S₅Se₃, (b) Cu₂Mo₆S₄Se₄ and acid leachedMo₆S₄Se₄, and (c) Cu₂Mo₆S₃Se₅ and acid leached Mo₆S₃Se₅, respectively,in accordance with certain embodiments of the invention;

FIG. 7a is a plot that shows galvanostatic cycling data and FIG. 7b is aplot that shows capacity versus voltage profiles obtained at a currentrate of 20 mAg⁻¹ cycled between 0.5-1.5V for Mo₆S₅Se₃ electrodes usingthe 0.4 molar 2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte, in accordancewith certain embodiments of the invention;

FIG. 8a is a plot that shows galvanostatic cycling data and FIG. 8b is aplot that shows capacity versus voltage profiles performed at a currentrate of 20 mAg⁻¹ between 0.5-1.5 V for Mo₆S₄Se₄ electrodes using the 0.4molar 2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte, in accordance withcertain embodiments of the invention;

FIG. 9a is a plot that shows galvanostatic cycling data and FIG. 9b is aplot that shows capacity versus voltage profiles performed at a currentrate of 20 mAg⁻¹ between 0.5-1.75 V for Mo₆S₃Se₈ electrodes using the0.4 molar 2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte, in accordancewith certain embodiments of the invention; and

FIG. 10 is a plot that shows rate capabilities of Mo₆S₅Se₃ and Mo₆S₄Se₄electrodes in a Mg-ion battery, in accordance with certain embodimentsof the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention relates to Chevrel-phase materials, such as ternarymolybdenum chalcogenide compounds. The Chevrel-phase materials, alsoreferred to as Chevrel-phase compounds, in accordance with the inventionhave the formula Mo₆Z₈, wherein Mo represents molybdenum and Zrepresents a chalcogen, or Mo₆Z¹ _(8-y)Z² _(y), wherein Mo representsmolybdenum and each of Z¹ and Z² represents a first chalcogen and adifferent second chalcogen, respectively. The invention further relatesto electrochemical cells incorporating a cathode which includes aChevrel-phase material, an anode which includes an alkali metal, and anelectrolyte.

The Chevrel-phase materials of the formula Mo₆Z₈ or Mo₆Z¹ _(8-y)Z² _(y)are prepared using various techniques and methods. In accordance withcertain embodiments of the invention, the Chevrel-phase materials arederived from said precursor materials. The precursor materials have theformula M_(x)Mo₆Z₈, wherein M is a metallic element, ‘x’ is a numberfrom greater than 0 to 4, and Z is a chalcogen, e.g., chalcogenide, orthe formula M_(x)Mo₆Z¹ _(8-y)Z² _(y), wherein M is a metallic element,‘x’ is a number from greater than 0 to 4, ‘y’ is a number from greaterthan 0 to less than 8, and each of Z¹, and Z² is a chacogen, e.g.,chalcogenide, wherein Z¹ is a different chalcogen than Z².

The said precursor materials of the formulas M_(x)Mo₆Z₈ or M_(x)Mo₆Z¹_(8-y)Z² _(y) are formed using various methods. In accordance withcertain embodiments of the invention, the precursor materials are formedby combining metal sulfides (MZ and MoZ₂, or MZ¹, MZ², MoZ₂ ¹ and MoZ₂²) with molybdenum (Mo). Further, in certain embodiments, the metalsulfides (MZ and MoZ₂, or MZ¹, MZ², MoZ₂ ¹ and MoZ₂ ²) and molybdenum(Mo) metal are subjected to high energy mechanical milling (HEMM) andduring the ensuing milling process, the individual materials mentionedreact resulting in forming the precursor materials.

In certain embodiments, the alkali metal is magnesium. Thus, theelectrochemical cells in accordance with certain embodiments of theinvention include a magnesium or magnesium-containing anode.

It is contemplated and understood that M includes a wide variety ofmetallic elements that are known to one having ordinary skill in theart. In certain embodiments, the metallic element is selected from Li,Na, Mg, Ca, Sc, Cr, Mn, Fe Co, Ni, Cu, Zn, Sr, Y, Pd, Ag, Cd, In, Sn,Ba, La, Pb, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu andmixtures thereof.

The chalcogens Z, Z¹ and Z² are selected from those known to one havingordinary skill in the art. In certain embodiments, the chalcogens Z, Z¹and Z² are selected from the chemical elements in Group 16 of thePeriodic Table, including sulfur, selenium, tellurium and mixturesthereof. Further, in certain embodiments, each of Z, Z¹ and Z² issulfur, selenium, tellurium or a mixture thereof, with or without thepresence of oxygen. Typically, the oxygen when present is in a small ortrace amount. As aforementioned, Z¹ is a first chalcogen and Z² is adifferent second chalcogen.

In accordance with certain embodiments of the precursor approach for usein preparing Chevrel-phase materials, stoichiometric amounts of MZ, MoZ₂and Mo, or stoichiometric amounts of MZ¹, MZ², MoZ₂ ¹, MoZ₂ ² and Mo arecombined to form a mixture. The mixture is heated to complete thedesired reaction or at least to substantially complete the desiredreaction. The heating can be accomplished by various conventionalmechanisms known in the art. For example, heating can be conducted overa hot plate at a temperature of about 90° C. under nitrogen bubbling.Following heating, the mixture is filtered using conventional techniquesknown in the art. Tetrahydrofuran (or any other known suitable solventmaterial) is added to the filtrate to initiate incipient precipitation.The resulting precipitate is washed in accordance with known techniques,e.g., with tetrahydrofuran and alcohol, e.g., methanol, and then driedusing known techniques, e.g., at a temperature of about 60° C.

In certain embodiments, the dried product is ground and heated in anargon (or the like) atmosphere at about 1000° C. to yield metallicChevrel-phase precursor material, e.g., M₂Mo₆Z₈ or M₂Mo₆Z¹ _(8-y)Z²_(y).

The metals, e.g., metallic ions, are leached out using conventionalmechanisms. For example, the metallic ions can be removed under anambient atmosphere using a solution of HCl/O₂ bubbling. The solution isthen centrifuged, washed and dried using known methods and techniques toyield the final Chevrel-phase product, e.g., Mo₆Z₈ or Mo₆Z¹ _(8-y)Z²_(y).

In certain embodiments, the starting materials for use in forming theprecursor material can be subjected to high energy mechanical milling(HEMM). Accordingly, appropriate amounts of MZ, MoZ₂ and Mo, orappropriate amounts of MZ¹, MZ², MoZ₂ ¹, MoZ₂ ² and Mo are charged intoa HEMM apparatus. The HEMM process is carried out for a defined periodof time which can vary. The time period can be as short in duration asthirty minutes or as long as one or two or three hours or even more asneeded not extending beyond few hours to initiate and complete thereaction. The HEMM process is subsequently followed by annealing at anelevated temperature under ultra high purity (UHP) argon atmosphere. Thetemperature can vary and may be maintained from about 1123 to about1273K.

An advantage of employing the HEMM process is that no vacuum or silicaampoule is required for the synthesis due to complete absence ofelemental sulfur/selenium in the starting composition. Further, there isno loss or gain in mass between the Chevrel-phase product and theprecursor reactants. The ternary Chevrel phase can be formed directlyfrom the ball-milled or mechanically milled powder upon annealing, amajor advantage of the invention. In certain embodiments, the annealingis carried out a temperature that is about 1073 K or greater in acorundum crucible in a tubular furnace.

In certain embodiments, M is copper, ‘x’ is 2 and Z is sulfur. Theprecursor material is Cu₂Mo₆S₈ which is formed by the combination andreaction of CuS, MoS₂ and Mo. The Chevrel-phase material has the formulaof Mo₆S₈ and is derived from the precursor material Cu₂Mo₆S₈ followed byremoving the copper or copper ions therefrom by subsequent acidleaching. Suitable materials for use in forming the precursor materialcan include a mixture of stoichiometric amounts of ammoniumtetrathiomolybdate and anhydrous copper chloride (CuCl₂) in the presenceof anhydrous N,N-dimethylformamide. For example, formation of theChevrel-phase material can be conducted according to the formula: 2CuS+3Mo+3MoS₂═Cu₂Mo₆S₈. For ease of description, much of the disclosureis directed to the chalcogen being sulfur (i.e., Z represents S) and themetallic element is copper (i.e., M represents Cu). However, it isunderstood that in accordance with the invention, the chalcogen is notlimited to sulfur and the metallic element is also not limited tocopper.

In certain other embodiments, M is copper, ‘x’ is 2, Z¹ is sulfur, Z² isselenium and ‘y’ is a number from greater than 0 to less than 8. Theprecursor material is Cu₂Mo₆S_(8-y)Se_(y) which is formed by thereaction of a combination of CuS, CuSe, MoS₂, MoSe₂ and Mo. TheChevrel-phase material has the formula of Mo₆S_(8-y)Se_(y) and isderived from the precursor material Cu₂Mo₆S_(8-y)Se_(y) followed byremoving the copper or copper ions therefrom by subsequent acidleaching. Suitable materials for use in forming the precursor materialcan include a mixture of stoichiometric amounts of ammoniumtetrathiomolybdate and anhydrous copper chloride (CuCl₂) in the presenceof anhydrous N,N-dimethylformamide. For example, formation of theChevrel-phase material can be conducted according to the formulas:Cu₂Mo₆S₈ (Cu₂ CP); Cu₁Mo₆S₈ (Cu₁ CP); Cu₂Mo₆S₇Se₁ (Cu₂ CPSe₁); Mo₆S₇Se₁;Cu₂Mo₆S_(8-x)Se_(x) (x=3, 4, 5), Cu₂Mo₆S₅Se₃ (Cu₂ CPSe₃); Mo₆S₅Se₃,Cu₂Mo₆S₄Se₄ (Cu₂ CPSe₄); Mo₆S₄Se₄, Cu₂Mo₆S₃Se₅ (Cu₂ CPSe₅); Mo₆S₃Se₅.For ease of description, much of the disclosure is directed to thechalcogens being sulfur and selenium (i.e., Z¹ represents S, and Z²represents Se) and the metallic element is copper (i.e., M representsCu). However, it is understood that in accordance with the invention,the chalcogens are not limited to only sulfur and selenium, and themetallic element is not limited to only copper. The Mo₆S_(8-y)Se_(y)units are linked with each other and form a three-dimensional frameworkwith open cavities/channels that can be filled with wide-variety ofguest atoms and form ternary CPs M_(x)Mo₆S_(8-y)Se_(y) (0<x<4 and0<y<8).

In general, the precursor approach includes preparing a precursormaterial which includes a metallic element and the metallic element (orions thereof) is subsequently completely or partially removed (e.g.,leached out) to produce the said Chevrel-phase product for use ascathode material.

The Chevrel-phase material synthesized in accordance with theabove-described precursor approach can be employed to prepare andfabricate an electrode, i.e., cathode, using conventional techniques andapparatus. For example, an electrode slurry containing the Chevrel-phasematerial of the invention can be prepared and deposited on a currentcollector, and then subsequently dried. The current collector caninclude a wide variety of suitable materials known by one havingordinary skill in the art. Non-limiting examples include but are notlimited to copper, graphite, nickel, platinum, quartz, gold, stainlesssteel, tantalum, titanium, silver, and mixtures thereof. In certainembodiments, the current collector is also graphite foil (grafoil).Deposition of the slurry can include forming a coating, e.g., thin film,on the current collector. Drying can be accomplished using conventionalmechanisms such as drying in a vacuum oven.

The Chevrel-phase cathode material can be combined with an alkali-metalanode, e.g., a magnesium or magnesium alloy-containing anode, and anelectrolyte to form an electrochemical cell, such as a magnesium ionrechargeable battery.

The electrolyte can be in the form of a solution. The electrolytesolution can include electrolyte salt containing magnesium and solvent.

In general, suitable electrolytes with high anodic/oxidative stabilityabove 3V wherein magnesium can be deposited reversibly are not widelyknown and available in the art. It has been found that magnesium may notbe reversibly deposited from solutions of simple magnesium salts, e.g.,Mg(ClO₄)₂, in conventional organic solvents, e.g., acetonitrile,propylene carbonate or N, N-dimethylformamide. This is primarily due tothe formation of a dense passivating surface/blocking layer on thereactive magnesium electrode surface by the reduction products. It isbelieved and well-known that bare magnesium metal reacts with anionssuch as ClO₄, BF₄ ⁻ to form insoluble magnesium salts/halides that blockthe magnesium electrodes. Moreover, the surface films effectively blockthe electrodes, as the mobility of the Mg²⁺ ions through the imperviouspassivating films is low. After a period of time, the capacity fadesleading to failure of the electrochemical cell.

It is generally known that magnesium can be deposited using Grignardreagents in ethereal solvents. Magnesium can be electro-depositedreversibly primarily based on the solutions of tetrahydrofuran (THF)using Grignard reagents (R—MgX, R=alkyl; X═Br, CO, and amidomagnesiumhalides. However, due to the strongly reducing character of Grignardreagents, and limited oxidative stability (EtMgBr and BuMgCl have anoxidative stability 1.5V vs. Mg), it appears to be incompatible orrealistically not possible for cathodes to be developed for use inbatteries. Also, it is known to that tetrahydrofuran (THF) or primaryamines (N-methylaniline) can be as electrolytes solvents in which bothmagnesium dissolution and deposition will occur. However, the Coulombicefficiency is typically low. In accordance with the invention,amidomagnesium halide-based Grignard reagent of the formula[3-[bis(trimethylsilyl)amino]phenylmagnesium chloride[(CH₃)₃Si]₂NC₆H₄MgCl] solution in 1.0 M THF is a suitable salt forsynthesis of magnesium battery electrolyte. Due to the presence of anaromatic ring coupled with an amino methylsilyl group with MgCl⁺ ion,and electron lone pair donated to the aryl phenyl group, the R—Mg bondis thus strong and likely precludes or prevents the oxidation of theanionic species.

In certain embodiments of the invention, the electrolyte solutionincludes amidomagnesium-based magnesium salt transmetallated with analuminum salt electrolyte dissolved in the solvent. In otherembodiments, the electrolyte solution includes phenyl magnesiumchloride-aluminum chloride, amidomagnesium-based magnesium salttransmetallated with an aluminum salt electrolyte, and solvent. Thesolvent can be tetrahydrofuran. In yet other embodiments, theelectrolyte solution includes 3-bis(trimethylsilyl)aminophenylmagnesiumchloride with aluminum chloride in tetrahydrofuran.

The cathodes, electrodes and electrochemical cells prepared inaccordance with the present invention demonstrate the desired benefitsand advantages over those that are known in the art. For example, theelectrochemical performance, e.g., rate capability and stable specificcapacity with galvanostatic cycling, of the products prepared accordingto the invention are found to be superior to the known products. TheChevrel-phase material prepared in accordance with the present inventionalso exhibits very high rate capability and cycling performance whenassembled in a rechargeable magnesium electrochemical battery cell.

EXAMPLES Example I

Materials Preparation

Cu₂Mo₆S₈ (CuCP) was synthesized as follows. In a 3-neck round bottomflask, stoichiometric amounts of ammonium tetrathiomolybdate (4 g, 15.37mmol; Alfa-Aesar 99.95%) and anhydrous copper(II) chloride (0.6890 g,5.12 mmol, Alfa-Aesar 99.985%) were added to N,N dimethylformamide, DMF(130 ml) solution. The resultant mixture was heated over a hot plate(˜90° C.) for 6 hours under constant N₂ bubbling. After completereaction, the mixture turned deep red in color and was filtered.Tetrahydrofuran (1:5 by volume) was added immediately to the filtrate toincipient precipitation. A fine black precipitate was formed. Thisprecipitate was kept overnight. Then, the precipitate was washed withtetrahydrofuran and methanol followed by drying at ˜60° C. for 24 hours.The final dried solid product was ground and heated in a UHP Ar+6.5% H₂atmosphere at ˜1000° C. for 5 hours which directly yielded copperChevrel phase Cu₂Mo₆S₈. The copper ions were leached out under anambient atmosphere using a solution of 6 M HCl/O₂ bubbling for 8 hoursaccording to the method outlined by Lancry, et al. (“Leaching Chemistryand the Performance of the Mo₆S₈ Cathodes in Rechargeable Mg Batteries,Chemistry of Materials”, 16 (2004) 2832-2838). Following complete copperleaching, the solution was centrifuged, washed with the same solvent anddried in an oven at ˜60° C. In order to perform qualitative phaseanalysis, Z-ray diffraction (XRD) was carried out using the PhilipsPW1830 system employing the CuK_(α) (λ=0.15406 nm) radiation.Microstructural analysis of the initial Cu₂Mo₆S₈ and Mo₆S₈ obtainedafter copper removal was then performed using a scanning electronmicroscopy (JSM-6610, JEOL) operating at 10 kV.

Electrochemical Characterization

An electrode slurry was prepared by mixing 80 wt. % of the Mo₆S₈ powder(˜325 mesh), 10 wt. % Super P, 10 wt. % polyvinylidene fluoride (PVDF)dissolved into a solution of N-methylpyrrolidinone (NMP) to make ahomogeneous solution. The slurry was coated onto a graphite foil anddried at ˜110° C. in a vacuum oven. Electrochemical characterization wasconducted at room temperature using fabricated electrodes assembled in2016 coin cells that were in turn assembled inside an argon-filledMBraun Inc. glove box (<0.1 ppm each of O₂ and H₂O) employing magnesiumfoil as a counter electrode and cellgard separator soaked in anelectrolyte solution of 0.4 molar 2(PhMgCl)—AlCl₃ in tetrahydrofuran.Cyclic voltammogram (CV) was performed using an electrochemicalworkstation (VersaSTAT 3, Princeton Applied Research) between 0.5-1.75 Vat a constant sweep rate of ˜0.01 mVs⁻¹. Galvanostatic charge-dischargecycles were carried out employing various current rates of 20-120 mA/gwithin 0.5-1.5 V, with a short rest period between the charge/dischargecycles using a multichannel battery testing system (Arbin BT2000instrument).

Results

The XRD patterns of the heat-treated powder obtained by the precursorroute shows characteristic XRD peaks. The major diffraction peaks wereindexed to the rhombohedral phase of Cu₂Mo₆S₈ (space group: R-3; number:148; JCPDS-ICDD: 00-047-1519). Similarly, the XRD pattern of the powderobtained after acid removal of copper ions matches with the rhombohedralMo₆S₈ (space group: R-3; number: 148; JCPDS number: 00-027-0319). Theexact match of the XRD patterns of pristine Cu₂Mo₆S₈ and acid leachedMo₆S₈ phase compared with the standard XRD pattern suggests that theprecursor route is a simple and convenient approach to the directsynthesis of Chevrel phase compounds. An impurity phase of MoS₂ (spacegroup: P63/mmc, number: 194, JCPDS number: 01-073 1508) was observed inboth Cu₂Mo₆S₈ and Mo₆S₈ identified by the presence of peaks at 2θ=14.4°,32.8°, and 39.6°. The SEM micrographs of the Cu₂Mo₆S₈ and Mo₆S₈ powdermaterials showed that distinct micrometer-size cuboidal shapecrystallites (˜0.5-2 μm) were formed. The crystallites were relativelysmaller for the Mo₆S₈ as compared to the Cu₂Mo₆S₈ phase.

Cyclic voltammogram at a scan rate ˜0.01 mVs⁻¹ has been recorded between0.5-1.75 V of the electrode (comprising 80% Mo₆S₈, 10% PVDF, 10%Super-P) in a 0.4 M THF/2(PhMgCl)—AlCl₃ electrolyte solution. The CVcurve indicates a highly reversible behavior for magnesium-ioninsertion/extraction in the Mo₆S₈ phase similar to the expectedelectrochemical phenomena based on observations in the art. The anodicand cathodic peaks observed at ˜1.05 V and 1.17 V respectively, suggestMg²⁺ insertion/extraction into the Mg_(x)Mo₆S₈ (0<x<2) phase. It isestablished that Mg²⁺ insertion into the Mo₆S₈, Chevrel phase occurs intwo stages, but due to partial charge entrapment after the initialmagnesiation only 60-80% magnesium-ion can be extracted during the firstcharge, resulting in 20-25% capacity loss compared to the theoreticalvalue (128 mAhg⁻¹). The loss in capacity may be salvaged if theelectrochemical cell can be operated at an elevated temperature (˜60-80°C.). The galvanostatic charge-discharge profiles (1^(st), 2^(nd),25^(th) and 50^(th) cycle) of the Mo₆S₈ electrode performed at aconstant current rate of ˜20 mAg⁻¹ (˜C/6). During first discharge,electrochemical Mg²⁺ insertion occurred into Mo₆S₈ host, and offered aspecific discharge capacity of ˜116 mAhg⁻¹ (91% of theoretical value).During the 1^(st) discharge curve, Mg²⁺ insertion plateaus occurred at˜0.9 V, whereas Mg²⁺ extraction occurred at ˜1.2 V. The 2^(nd), 25^(th),and 50^(th) cycles indicated only single reaction plateau at ˜1.1 V(Mg²⁺ insertion) and at ˜1.2 V (Mg²⁺ extraction), respectively. Thecycling data suggested that the initial Mg²⁺ insertion into Mo₆S₈ wasdifficult and intrinsically very slow requiring a slight overvoltage of200 mV compared to the subsequent cycles. The 1^(st) cycle discharge(˜116 mAhg⁻¹) and charge capacity (˜104 mAhg⁻¹) suggested 91%magnesiation and 81% demagnesiation occurred from the Mo₆S₈ host(resulting in a 1^(st) cycle irreversible loss 10.3%). Complete removalof Mg²⁺ ion was not possible at room temperature due to the partialcharge trapping in the Mo₆S₈ host. The variation of specific capacityvs. cycle number along with coulombic efficiency of the Mo₆S₈ electrode,cycled at a constant current of ˜20 mAg⁻¹ (˜C/6 rate) in the potentialwindow of 0.5-1.5 V shows that the 1^(st) cycle discharge and chargecapacity of the Mo₆S₈ electrode is ˜116 mAhg⁻¹ and 104 mAhg⁻¹,respectively, with a 1^(st) cycle irreversible loss (FIR) of ˜10.3%.However, from the 2^(nd) to the 50^(th) cycle, it is shown that therewas a steady charge and discharge capacity of ˜80 mAhg⁻¹ and ˜76 mAhg⁻¹respectively, which resulted in an improved coulombic efficiency of˜95%. This excellent capacity retention may be ascribed to the formationof high surface area cuboidal shaped ˜0.5-2 μm, Mo₆S₈ particles, whichallowed suitable wetting of the active material with the electrolytepromoting good charge transfer. The nd, 25^(th), differential capacityplot (dQ/dV vs. V) of the 1^(st), 2^(nd), 25^(th), 50^(th) cycle showsthat during the 1^(st) discharge and charge cycles,magnesiation/demagnesiation occurred at ˜0.92 V and at ˜1.17 V,respectively. A sharp peak is shown at ˜1.10 V (magnesiation) and at˜1.17 V (demagnesiation) for the 2^(nd), 25^(th), and 50^(th) cycleswhich was due to the known phenomena of partial entrapment of Mg²⁺ ionafter the 1^(st) magnesiation (Mg₂Mo₆S₈) reaction which resulted in onlyone Mg²⁺ ion cycling into/from the Mg_(x)Mo₆S₈ (0<x≤1) host from the2^(nd) cycle and thereon. The differential capacity plot matched wellwith the charge-discharge profile of the electrode cycled at 20 mAg⁻¹which suggested excellent cyclability and reversibility of Mg²⁺insertion/extraction phenomena of the as-synthesized Mo₆S₈ electrode inan Mg cell.

The excellent electrochemical rate performance was validated. TheMg/Mo₆S₈ cell was cycled at various current rates. The results showedthe rate capability of the Mo₆S₈ electrode performed at current rates of20 mAg⁻¹ (˜C/6), 64 mAg⁻¹ (˜C/2), 128 mAg⁻¹ (˜C), and 192 mAg⁻¹ (˜1.5C). The capacity retention of the synthesized cuboidal Mo₆S₈ via thenovel precursor approach was excellent compared to literature reports ofsimilar Chevrel phase compounds synthesized by published approaches. Thedischarge capacities at the above current rate was ˜76 mAhg⁻¹, ˜72mAhg⁻¹, ˜68 mAhg⁻¹, and ˜66 mAhg⁻¹ respectively. The coulombicefficiency at ˜/6, ˜/2, ˜1 C, and ˜1.5 C rate was 95%, 97.8%, 98.9%, and99.3%, respectively. Although it is expected that coulombic efficiencydecreases with increasing C-rate, the unique features of cuboidal Mo₆S₈(excellent electronic conductivity) rendered it suitable for fastinsertion/extraction of Mg²⁺ ions at ambient temperature with animproved coulombic efficiency in a Mg prototype cell.

Conclusions

In summary, unique cuboidal shape ˜0.5-2 μm size Cu₂Mo₆S₈/Mo₆S₈ Chevrelphase was synthesized by a rapid and facile precursor route. XRDconfirmed the phase formation and the electrochemical measurementsindicated superior performance, such as rate capability and stablespecific capacity. The Mo₆S₈ Chevrel phase exhibited extremely high ratecapability and cycle performance when assembled in an Mg cell. The celldelivered a capacity of ˜66 mAhe at ˜1.5 C rate making it suitable as acathode for a magnesium-ion battery.

Example II

Stoichiometric amounts of MoS₂, Mo, and CuS were batched in a SS vial(powder: ball ratio=1:10). The powders were mechanically milled forintervals of 1 hour, 2 hours and 3 hours, and subjected to XRD patterns.After 3 hours of milling, the powder was heat-treated at 1000° C./5hours under UHP argon atmosphere. The X-ray diffraction patterns showedthe synthesis of Cu₂Mo₆S₈ Chevrel-phase by the high energy mechanicalmilling route. The XRD patterns of the heat-treated powder also showedthe formation of pure crystalline Cu₂Mo₆S₈. The Cu₂Mo₆S₈ was washed withHCL/O₂ bubbling for 7 hours to yield completely crystalline Mo₆S₈.

Example III

Stoichiometric amounts of MoSe₂, Mo, and CuSe were batched in a SS vial(powder: ball ratio=1:10). The powders were mechanically milled forintervals of 1 hour, 2 hours and 3 hours, and subjected to XRD patterns.After 3 hours of milling, the powder was heat-treated at 1000° C./5hours under UHP argon atmosphere. The XRD analysis and patternscollected showed the synthesis of Cu₂Mo₆Se₈ Chevrel-phase by the highenergy mechanical milling route. Further, the XRD patterns of theheat-treated powder showed the formation of pure crystalline Cu₂Mo₆Se₈.The Cu₂Mo₆Se₈ was washed with HCL/O₂ bubbling for 7 hours to yieldcompletely crystalline Mo₆Se₈.

Example IV

Novel amidomagnesium-based electrolytes at varying molar ratios werecompared with Aurbach's Pt and 2^(nd) generation electrolytes byconducting cyclic voltammetry. The electrochemical parameters obtainedfrom the cyclic voltammograms conducted at a 25 mvs⁻¹ scan rate usingamidomagnesium-based novel electrolytes at different molar ratios arecompared with Aurbach's 1^(st) generation electrolyte (i.e., 0.25 molarMg(AlCl₂EtBu)₂/THF) and 2^(nd) generation electrolyte (i.e., 0.4molarPhMgCl)—AlCl₃/THF) are shown in Table 1.

TABLE 1 Electrochemical parameters obtained from the cyclicvoltammograms conducted at 25 mVs⁻¹ scan rate using novel amidomagnesiumbased electrolytes at different molar ratio and compared with Aurbach's1^(st) and 2^(nd) generation electrolyte. Onset Anodic base/aciddeposition stripping peak Mg cycling ratio potential (V) potential (V)efficiency (%) 1:1 −0.283 0.983 93.2 2:1 −0.242 0.842 63.7 3:1 −0.3510.401 56.5 1:2 (1^(st) −0.193 1.032 95 generation) 2:1 (2^(nd) −0.370.643 85.5 generation)

The 1:1 (Lewis base: Lewis acid ratio) electrolyte conducted at a scanrate of 25 mVs⁻¹ demonstrated enhanced performance for all of the novelelectrolytes in accordance with the invention. Further, Table 1 suggeststhat the coulombic efficiency for reversible magnesiumdeposition/dissolution was near or about 93% for the 1:1 electrolyte.The coulombic efficiency for the 2:1 and 3:1 electrolytes was about 64%and about 56%, respectively.

The linear sweep voltammograms obtained at a scan rate of 1 mVs⁻¹ fromopen circuit potential to approximately 4 V showing the anodic stabilityonto Pt working electrode of the novel amidomagnesium-based electrolytesat varying molar ratios as compared with Aurbach's 1^(st) and 2^(nd)generation electrolyte. The linear sweep voltammograms of theabove-mentioned electrolytes from open circuit potential to about 4 V at1 mVs⁻¹ showed that the electrochemical stability of theamidomagnesium-based novel electrolytes of 1:1, 2:1 and 3:1 was about2.27, about 2.33 and about 2.19 V, respectively, onto noble metal (e.g.,platinum electrode). The electrochemical stability of the novelelectrolyte was similar to Aurbach's 1^(st) generation electrolyte butsignificantly lower than Aurbach's 2^(nd) generation electrolyte.

Example V

A Mo₆S₈ cathode was tested in a 2016 coin cell using 0.4 M3((3-bis(trimethylsilyl)amino)phenylmagnesium chloride)-AlCl₃THF (3:1novel electrolyte). The electrochemical performance in particular whenconducted at a current rate of 20 mAg⁻¹ and cyclic voltammetry employinga scan rate of 100 microvolts per second (100 μVs⁻¹) showed a 1^(st)cycle irreversible loss of about 40% and stable discharge capacity about37 mAhg⁻¹ at C/6 rate with coulombic efficiency about 90%.

Example VI

A Mo₆S₈ cathode was tested in a 2016 coin cell using 0.4 M2((3-bis(trimethylsilyl)amino)phenylmagnesium chloride)-AlCl₃THF (2:1novel electrolyte). The electrochemical performance in particular whenconducted at a constant current of 20 mAg⁻¹ and a scan rate of 100microvolts per second (100 μVs⁻¹) showed a 1^(st) cycle irreversibleloss of about 48% and stable discharge capacity about 54 mAhg⁻¹ at C/6rate with coulombic efficiency about 88%.

Example VII

A Mo₆S₈ cathode was tested in a 2016 coin cell using 0.4 M((3-bis(trimethylsilyl)amino)phenylmagnesium chloride)-AlCl₃THF (1:1novel electrolyte). The electrochemical performance in particular whencycled at a constant current of 20 mAg⁻¹ and a scan rate of 100microvolts per second (100 μVs⁻¹) showed a 1^(st) cycle irreversibleloss of about 26% and stable discharge capacity about 60 mAhg⁻¹ at C/6rate with coulombic efficiency about 99%.

Example VIII

Cu₂Mo₆S₈ (Cu₂ CP) Chevrel phase was developed by High Energy MechanicalMilling (HEMM) route. Stoichiometric amounts of MoS₂, Mo, and CuS werebatched in a SS vial (powder: ball ratio=1:10). The powders weremechanically milled for 1 hour, 2 hour, and 3 hour intervals andsubjected to XRD analysis to collect the XRD patterns. After 3 hours ofhigh-energy mechanical milling (HEMM), the powder was heat-treated at1000° C./5 h under UHP Ar atmosphere. XRD pattern of the heat-treatedpowder collected showed the formation of pure crystalline Cu₂Mo₆S₈.Further, Cu₂Mo₆S₈ was washed with HCl/O₂ bubbling for 7 hours yields toobtain completely crystalline Mo₆S₈ (CP).

It was established that Mg²⁺ insertion into Mo₆S₈ Chevrel phase occurredin two stages, and theoretically could offer a capacity ˜128 mAhg⁻¹.However, due to partial charge entrapment after initial magnesiation,only 50-60% magnesium-ion could be extracted during first charge,resulting in 40-50% irreversible loss in the 1^(st) cycle from thetheoretical value (˜128 mAhg⁻¹). In order to minimize the 1^(st) cycleirreversible loss, Cu was partially leached from the original Cu₂ CPstructure. 1.8 g of Cu₂ CP was added in 20 ml 6M HCl solution in a smallglass vial with a magnetic stir bar. Cu₂ CP/HCl solution wascontinuously stirred for 2 days at room temperature. After 2 days ofcontinuous stirring, the solution was ultrasonically cleaned usingdistilled water (3 times) and dried at a temperature of 60° C. for 24hours. XRD pattern of the partial leached Cu₂ CP collected showed theformation of completely crystalline partially de-cuprated, CuMo₆S₈ (Cu₁CP).

Results

The XRD patterns were collected on Cu₂Mo₆S₈ (˜Cu₂ CP) powder obtained byhigh energy mechanical milling of CuS, MoS₂ and Mo stoichiometriccompositions followed by heat treatment at 1000° C./5 h under UHP Ar,and acid leached CuMo₆S₈ (CuCP) obtained after 2 days partial leachingof copper using 6M HCl solution. The Bragg diffraction lines wereindexed to a hexagonal-rhombohedral symmetry unit cell of Cu₂Mo₆S₈(space group: R-3; number: 148; JCPDS-ICDD: 00-047-1519). Latticeparameter(s) calculated using least-square method of the experimentaldata (a=0.96478 nm, c=1.02026 nm, and unit cell volume=822.42×10⁻³ nm³)was in good agreement with the standard Cu₂Mo₆S₈ unit cell parameters(a=0.9584 nm, c=1.025 nm, unit cell volume=815.36×10⁻³ nm³). Similarly,the XRD pattern obtained after removal of one copper from theheat-treated powder using hydrochloric acid treatment matched with theJCPDS patterns. The Braggs lines were indexed with thehexagonal-rhombohedral symmetry unit cell of CuMo₆S₈ phase (space group:R-3; number: 148; JCPDS-ICDD: 00-034-1379), and the calculated latticeparameter(s) (a=0.94412 nm, c=1.04761 nm, and unit cellvolume=808.70×10⁻³ nm³) matched quite well with standard unit cellparameters of CuMo₆S₈ obtained from above ICDD database (a=0.94120 nm,c=1.04070 nm, unit cell volume=798.40×10⁻³ nm³). The calculated latticeparameters values were consistent with the standard lattice parameter(s)values of pristine Cu₂Mo₆S₈ and acid leached CuMo₆S₈ powder obtainedfrom JCPDS-ICDD database suggested that 6M HCl treatment for 2 daysleached 50% copper from the original Cu₂ CP structure and yieldedCuMo₆S₈ (CuCP).

The variation of specific capacity versus cycle number along withcoulombic efficiency of the CuMo₆S₈ electrode, cycled at a constantcurrent of 20 mAg⁻¹ (˜C/6 rate) in the potential window of 0.5-1.5 Vusing 0.4 molar 2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte showed thatthe 1^(st) cycle discharge and charge capacity of the CuMo₆S₈ electrodewas ˜105 mAhg⁻¹ and ˜78 mAhg⁻¹, respectively, with a 1^(st) cycleirreversible loss (FIR) of ˜25.7% (or coulombic efficiency of ˜74.3%).It is to be noted that, the 1^(st) cycle irreversible loss (FIR) ofMo₆S₈ is ˜50% obtained by completely leaching out copper from the HEMMderived Cu₂ CP structures. Further, from 10^(th) cycles onward there wasa steady charge-discharge capacity of ˜55 mAhg⁻¹, with a coulombicefficiency of ˜99.9%. The preliminary results of CuCP suggested thatpartial Mg²⁺ charge entrapment which is common during 1^(st) cyclemagnesiation (discharge) in Mo₆S₈ cathode may be overcome with CuMo₆S₈structure where one Mg²⁺ can cycle without any hindrance.

The HEMM derived Mo₆Se₈ cathode assembled in a 2016 coin cell using Mgmetal anode, 0.4 molar 2(PhMgCl)—AlCl₃/tetrahydrofuran electrolyteexhibited a ˜17% capacity fade after 100 cycles. The exact reason offading is still under investigation. However, in order to improve theelectrical conductivity of the Mo₆Se₈ cathode, it was mixed withconductive graphite and coated with Mg²⁺ conducting MgSO₄ and used ascomposite cathode.

Example IX

The Cu₂Mo₆Se₈ (˜Cu₂ CP) Chevrel phase developed by 3 hours of HighEnergy Mechanical Milling (HEMM) of stoichiometric amounts of MoSe₂, Mo,and CuSe was followed by heat-treatment at 1000° C./5 h under UHP Aratm, were further mixed with commercial available synthetic graphite(SigmaAldrich, 1-2 m) in situ during milling process at 70:30 ratio(weight percent) and used as composite cathode for magnesium battery.Further, Cu₂Mo₆Se₈/graphite was washed with HCl/O₂ for 7 hours todevelop Mo₆Se₈/graphite composite cathode.

Example X

Mo₆Se₈ Chevrel-phase obtained after the complete removal of copper fromHEMM derived Cu₂Mo₆Se₈ was mixed thoroughly with previously preparedwater solution of commercially available MgSO₄ (Aldrich, 99.9%) at 70:30ratio (weight percent). Further the mixed slurry was dried at 60° C. for24 hours and used as MgSO₄ coated Mo₆Se₈ cathode for magnesium battery.

Example XI

Electrochemical impedance spectroscopy (EIS) was carried out on a Mo₆S₈electrode before and after cycling to evaluate Mg²⁺-ion charge storagemechanisms. EIS was performed using a Versastat 3 (Princeton AppliedResearch Inc.) potentiostat over a frequency range of 0.01 Hz-100 KHz.An A.C. amplitude of 5 mV was used and the spectra were obtained beforecycling and subsequent to 1^(st) and 2^(nd) discharge cycle. Electrodepotential was stabilized after discharge cycle and EIS was performed(i.e., immediately such that no significant relaxation processes andequilibrium phase changes occurred) to observe any noticeable changes inthe charge storage mechanism upon cycling. The effect of cycling oncharge transfer parameters was also analyzed by performing EIS. Z-View(Scribner Associates, Inc.; version 3.3) software was used for Randall'sequivalent circuit modeling of the experimentally collected impedancespectra.

Results

Electrochemical impedance study (EIS) was performed on a newly preparedMo₆S₈ electrode (wet chemical route) as well as cycled Mo₆S₈ electrodeat the magnesiated (discharged) and de-magnesiated (charged) statesassembled in a 2016 coin cell using magnesium foil as the counter andreference electrodes, Mo₆S₈ composite electrode as the working electrodeand 0.4 molar 2(PhMgCl)—AlCl₃ in tetrahydrofuran as the electrolyte. ANyquist plot was generated for the electrode prepared using the Chevrelphase (Mo₆S₈) at different stages. A clear transition in behavior wasobserved upon cycling. The impedance behavior was modeled using aRandall's circuit which considered a number of electrochemical phenomenaincluding:

-   -   a) High frequency series resistance (R_(s));    -   b) High frequency semi-circle (HFS) due to sluggish charge and        electron transfer kinetics (CPE_(e) and R_(e));    -   c) Gerischer impedance as a result of the coupling of a chemical        and an electrochemical process (GE);    -   d) Low frequency semi-circle (LFS) due to Mg-ion trapping in the        Chevrel phase (Mo₆S₈) host (CPE_(i) and R_(i)); and    -   e) Diffusion characteristic and associated ion-trapping (W_(o),        R_(trap), CPE_(trap)).

A model including all of these elements can be used for circuitmodeling. The model employed considered both charge transfer and masstransfer phenomena contributing to the electrode impedance. The circuitalso included constant phase elements (CPEs) due to the porous nature ofthe electrodes. In addition, it was found that the Gerischer impedanceelement (GE) was very small and insignificant for the electrode.

With respect to the effect of insertion/extraction on the impedanceprofile of the electrode materials, it was observed that the materialexhibited a significant increase in overall impedance following theinitial Mg²⁺-ion insertion after the 1^(st) discharge cycle. Table 2depicts the values of resistances obtained as a result of Randall'scircuit modeling. The solution resistance (R_(s)) was almost invariable.The charge transfer resistance (R_(e)) which is a characteristic ofelectron and ion transfer differed, pre- and post-Mg²⁺-ion insertionstage. There was a drop in R_(e) post Pt discharge and an increase fromsubsequent to 2^(nd) discharge. Interfacial resistance increased post Ptdischarge and decreased subsequent to 2^(nd) discharge. Charge trapping(R_(trap)) which remained very small before cycling and post Ptdischarge, increased by almost 8 orders, post 2^(nd) discharge. Thetransition in these three parameters provided information with respectto the Mg²⁺ charge storage mechanisms occurring in the Chevrel phase.The significant increase in overall impedance subsequent to firstdischarge indicated that the Mg²⁺-ion insertion was accompanied by arate limiting phenomenon. This, in turn, was seen by the two order risein interfacial resistance (R_(i)) subsequent to 1^(st) discharge. Theelectrode resistance, however, maintained the same order indicating thatpost-insertion in the 1^(st) discharge cycle, the electrode becamekinetically limiting for electrochemical charge transfer processes as aresult of the formation of either a secondary interface or a phasechange. After the 2^(nd) discharge, however, the R_(i) was small butR_(trap) and R_(e) increased indicating that part of the layer waseither inactive or became a barrier to solid state diffusion of Mg²⁺.Without intending to be bound by any particular theory, this masstransfer limitation and loss in magnesium trapped irreversibly as aresult of the phase change occurring in the 1^(st) discharge cycle, wasbelieved to be the reason for the low capacity occurring from the 2^(nd)cycle onward.

TABLE 2 Charge transfer parameter(s) of the Chevrel phase electrode atdifferent stages obtained from Randall circuit model. CPE_(e)- CPE_(e)-CPE_(i)- CPE_(i)- GE- GE- W_(o)- W_(o)- W_(o)- CPE_(trap)- CPE_(trap)-R_(s) T P R_(e) T P R_(i) T P R T P R_(trap) T P Before 10.27 8.03E−060.974 286.4 8.49E−06 1.009 53.28 0.693 0.134 5.041 4.22E−06 0.3932.32E−04 1.318E−04 0.125 cycling After 12.36 4.88E−06 0.941 182.67.23E−05 0.821 9748 5.195 0.089 0.205 1.61E−07 0.416 3.01E−05 1.287E−040.294 1^(st) dis- charge After 12.94 4.29E−05 0.76 921.2 1.14E−05 0.88165.09 3.849 0.728 0.135 1.08E−07 0.375 8855 5.813E−04 0.479 2^(nd) dis-charge

Example XII

The electrochemical behavior and stability of amidomagnesium-based threenon-aqueous electrolyte 0.4 molar [(CH₃)₃Si]₂NC₆H₄MgCl]—AlCl₃/THF wereinvestigated using both a three-electrode electrochemical cell and coincells. The cyclic voltammetry, linear sweep voltammetry, and coin cellcharge/discharge measurements indicated that the electrolyte is capableof reversibly cycling magnesium with a Coulombic efficiency ˜90% andelectrochemical stability ˜2.3V. Reversible cycling of magnesium fromthe amidomagnesium-based electrolytes were proved by scanning electronmicroscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX)analyses. SEM and EDX show that uniform spherical magnesium particles(1-5 μm) free from any dendrites formation deposited on platinumsubstrate. Moreover, Mo₆S₈ cathode derived by molten salt and highenergy mechanical milling route able to deliver a first cycle dischargecapacity ˜120-128 mAhg⁻¹ demonstrate the feasibility of the system aspotential 2V magnesium battery electrolyte.

1 molar [(CH₃)₃Si]₂NC₆H₄MgCl] salt in THF solution was transmetallatedwith 1 molar AlCl₃/THF in-house solution at three different Lewisbase-acid molar ratio of 1:1, 2:1 and 3:1 and studied theelectrochemical behavior using cyclic voltammetry, chronopotentiometry,linear sweep voltammetry, and galvanostatic charge-discharge cycles.Electrochemical data shows that amiodomagnesium-based electrolyte iselectrochemically stable up to 2.3V and capable of delivering 100%theoretical capacity (˜128 mAhg⁻¹) from Mo₆S₈ Chevrel phase cathode atroom temperature.

Experimental Section

Synthesis of 0.4 Molar L⁻¹ [(CH₃)₃Si]₂NC₆H₄MgCl]—AlCl₃/THF Electrolyte

The starting chemical of [(CH₃)₃Si]₂NC₆H₄MgCl] (1 molar intetrahydrofuran) was purchased from Sigma Aldrich and used withoutfurther purification. Highly pure inhibitor-free anhydroustetrahydrofuran (THF) was obtained from EMD Millipore and further dried(using Na chips and benzophenone) under ultra high purity Argonatmosphere using stringent drying steps outlined in the literature.Anhydrous AlCl₃ (99.999%) powder in glass ampoules was obtained fromSigma Aldrich and used as-received. First, 1 molar AlCl₃/THF wasprepared where ˜2 g AlCl₃ was gradually added in an anhydrous THF (˜15ml) through a powder addition funnel inside a Schlenk flask underconstant stirring. The above reaction is exothermic and a lightyellowish solution was obtained after the reaction was over. Theelectrolyte of chemical formula 0.4 molar L⁻¹[(CH₃)₃Si]₂NC₆H₄MgCl]—AlCl₃/THF at different molar ratio was synthesizedaccording to the following method. Stoichiometric amounts of[(CH₃)₃Si]₂NC₆H₄MgCl] (1 molar in THF) and AlCl₃ (1 molar in THF) (1:1,2:1, and 3:1 Lewis base/Lewis acid ratio in volume) were added in a 25ml clear glass vial. The mixture was continuously stirred for 2 h insidethe glass vial closed with a screw threaded cap. After 2 h reaction, thescrew cap was carefully opened and the electrolyte solution was dilutedusing anhydrous tetrahydrofuran (THF), resulting in the formation of 0.4molar L⁻¹ [(CH₃)₃Si]₂NC₆H₄MgCl]—AlCl₃/THF electrolyte. The electrolytepreparation was carried out inside an MBraun Inc. glove box where theoxygen and moisture levels are always maintained <0.1 ppm. 0.25 molarL⁻¹Mg(AlCl₂EtBu)₂/THF (butyl-ethyl complex or BEC) as well as 0.4 molarL⁻¹2 (PhMgCl)—AlCl₃/THF (all-phenyl complex or APC) were alsosynthesized as reference electrolytes according to the standard protocoloutlined in literature.

Characterization

Electrochemical analyses, including linear sweep voltammetry (LSV) andcyclic voltammetry (˜CV) were carried out on a CHI660D electrochemicalstation. The electrochemical cell used in the LSV and CV study was athree electrode cell which consisted of a working electrode (Pt),counter electrode (Mg), and reference electrode (Mg). The scan speed ofthe test was set to 10, 25, and 50 mV/s within −1V to +2.2V potentialranges. Deposition/dissolution efficiency of Mg on to Pt electrode wascarried out in 2016 coin cells at a constant current density of 0.25mAcm⁻² for 1 hour deposition and −0.25 mAcm⁻² for 1 hour dissolution forchronopotentiometic measurements. The three electrode cells wereassembled with the Pt working electrode, woven fiberglass (GFDseparator), Mg counter and reference electrode, and 0.4 M L⁻¹[(CH₃)₃Si]₂NC₆H₄MgCl]—AlCl₃/THF electrolyte.

Scanning electron microscopy (SEM) and energy-dispersive X-rayspectroscopy (EDX) were performed using a Philips XL-30 operating at anaccelerating voltage of 20 kV. It is noted that the electrodes werewashed by copious amount of THY three times and dried in vacuum beforeSEM and EDX analysis. The conductivity of the synthesized electrolytewas measured by a portable conductivity meter (HI991301, HANNA).

Results and Discussions

Amidomagnesium halide-based Grignard reagent of the formula[3-[bis(trimethylsilyl)amino]phenylmagnesium chloride[(CH₃)₃Si]₂NC₆H₄MgCl] solution in 1.0 THF was identified as a potentialsalt for the synthesis of ma magnesium battery electrolyte. Thechronopotentiogram of the 1^(st) cycle plating and stripping on platinumsubstrate using 1 mol L⁻¹ [(CH₃)₃Si]₂NC₆H₄MgCl]/THF solutionsdemonstrated the expected reversibility of Mg²⁺. Metallic magnesium wasreversibly plated onto platinum and stripped versus Mg²⁺/Mg couple in athree electrode cell at a constant current rate of ˜0.1 mA/cm² for 1 h.It was observed that the overpotential for magnesium plating (−0.123Vversus Mg²⁺/Mg) and stripping (0.103V versus Mg/Mg²⁺) was low andCoulombic efficiency ˜75% was observed suggest that the[(CH₃)₃Si]₂NC₆H₄MgCl]/THY Grignard reagent based solvent is capable ofcycling Mg-ion in a reversible manner. Cyclic voltammetry study of thisamiodomagnesium-based solvent using platinum as working electrode andmagnesium being counter and reference electrode in a three electrodecell further prove our hypothesis. The cell was first run from opencircuit potential to −1V (for Mg deposition) followed by −1V to 2.2V(for Mg stripping) using different voltage scan rate. The cyclicvoltammogram at different scan rates (10, 25 and 50 mV/s) between −1 to2.2 V showed the obvious reversible magnesium deposition/dissolutionwhich occurred within the potential window of ˜2.2V. The onset potentialfor 1^(st) cycle Mg deposition was −0.192V, −0.243V and −0.293V for 10mVs⁻¹, 25 mVs⁻¹ and 50 mVs⁻¹ scan rate respectively (see Table 1). Theincrease in onset potential with increasing voltage scan rate is due tosolution resistance and increasing kinetic barriers with high chargetransfer rate that exist in any standard electrochemical cell. A typicalcharge balance during cathodic plating and anodic stripping of the 1 molL⁻¹ [(CH₃)₃Si]₂NC₆H₄MgCl]/THF solutions showed 65% cyclic efficiency.Table 1 shows the onset deposition potential and the magnesium cyclicefficiency of the 1 mol L⁻¹ [(CH₃)₃Si]₂NC₆H₄MgCl]/THF solutions atdifferent scan rate. It appears that the 1 mol L⁻¹[(CH₃)₃Si]₂NC₆H₄MgCl]/THF Grignard reagent has some potential benefitslike the electrochemical stability window 1.5V but the Coulombicefficiency is very low (˜65%). However, preliminary electrochemical datasuggest that [(CH₃)₃Si]₂NC₆H₄MgCl]/THF is a Grignard reagent based Lewisbase is a promising system which can be transmetallated with a strongLewis acid (AlCl₃, BF₃ etc.) in order to improve its electrochemicalperformance as Aurbach et al. mentioned in literature. This prompted ourinterest to synthesize amidomagnesium-based electrolyte bytransmetallation of the amidomagnesium chloride [(CH₃)₃Si]₂NC₆H₄MgCl]with a strong Lewis acid (AlCl₃) at different base/acid ratio andfurther dissolving the reaction product in tetrahydrofuran at differentmolar level and study their electrochemical performance.

Three different electrolytes were synthesized at three differentbase/acid ratio (we will call it 1:1, 2:1, and 3:1 electrolyte) aslisted in Table 3.

TABLE 3 Electrochemical parameters of amidomagnesium based electrolytecompared with BEC and APC electrolyte including the anodic stability ofthe electrolytes on Pt working electrode. Anodic Onset stripping Anodicdeposition peak Mg cycling stability base/acid potential potentialefficiency limit in Pt ratio (V) (V) (%) (1 > 100 μAcm⁻²) 1:1 −0.2830.983 93.2 2.27 2:1 −0.242 0.842 63.7 2.33 3:1 −0.351 0.401 56.5 2.191:2 (BEC −0.193 1.032 95 2.2 (known) electrolyte) 2:1 (APC −0.37 0.64385.5 3.0 (known) Electrolyte

In order to validate the quality of the amidomagnesium-based threedifferent electrolytes, it was first evaluated by cyclic voltammetry(˜CV) study using a three-electrode cell. The first five cycles of CVwithin the potential window −1 V to 2.2V performed using platinumworking electrode, magnesium counter and reference electrode in 0.4M L⁻¹[(CH₃)₃Si]₂NC₆H₄MgCl]—AlCl₃/THF electrolyte family demonstrated thatdeposition and dissolution of magnesium in the present system is highlyreversible with a 1^(st) cycle deposition-dissolution efficiency of 93%for 1:1 electrolyte (Table 1). On the contrary, 1^(st) cycle magnesiumdeposition-dissolution efficiency was ˜63% and ˜56% for 2:1 and 3:1electrolytes, respectively. From the 2^(nd)-5^(th) cycles magnesiumdeposition/dissolution efficiency increased to ˜77% and ˜85% for 2:1 and3:1 electrolytes, respectively. The cycling efficiency for magnesiumdeposition-dissolution was calculated from total charge balance duringreduction/oxidation for each half-cycle as described in an earlierpublication. The overpotential for the deposition was observed to be−0.23V and remain steady from the 1^(st) to 5^(th) cycles for 1:1electrolyte. In the case of 2:1 and 3:1 electrolytes, overpotential forthe deposition was −0.24V and −0.35V respectively. The decrease inoverpotential and the increasing deposition-dissolution efficiency ofmagnesium from the 2^(nd) cycle onwards may be ascribed to thedesorption of electrolyte on the working electrode. Importantly, acyclic voltammetry study showed a minimum 5-fold increase in totalcurrent density for magnesium deposition and dissolution which suggestthat transmetallation of amidomagnesium-based Grignard reagent withAlCl₃ increases number of active ionic-species in the electrolytes forMg-ion transport.

The magnesium deposition and dissolution reactions on platinum substratecould be repeated hundreds of times for 1:1, 2:1, and 3:1 electrolytesas confirmed for 0.4 molar [(CH₃)₃Si]₂NC₆H₄MgCl]—AlCl₃/THF (1:1)electrolyte solution. The first three cycles for magnesiumdeposition-dissolution obtained from 1:1, 2:1 and 3:1 electrolytes at agiven current rate of 0.25 mAcm⁻² for 15 minutes (0.225 Ccm⁻²),respectively, showed that the overpotential for 1^(st) cycle depositiondrops drastically to a lower voltage from open circuit potential,however, the voltage steadied after 10-15 seconds. For 1:1, 2:1, and 3:1electrolyte, the overpotential for the deposition were −0.15V, −0.06Vand −0.07V for the first three cycles and dissolution were 0.1V, 0.05V,and 0.04V respectively. Low overpotential for magnesiumdeposition-dissolution observed from the chronopotentiogram suggestedthat the magnesium-ion transport was amenable in the above electrolytes.Galvanostatic cycling for the first 100 cycle magnesiumdeposition-dissolution on the platinum substrate gave a reoxidationcycling efficiency of 89% and 83%, respectively, for 1:1 and 2:1electrolytes. The reoxidation cycling efficiency of 3:1 was found tofluctuate between 60-80% for the first 50 cycles and then steadilyincreased to 88%. In all the three cases, the reoxidation cyclingefficiency was found to increase steadily from the 1^(st) to 20^(th)cycle may be due to desorption of electrolyte and electroactive specieson the working electrode. In order to confirm the deposit structure,magnesium was electrodeposited on a platinum substrate from the threedifferent amidomagnesium-based electrolytes at 0.5 mAcm⁻² for 1 hour(1.8 Ccm⁻²). SEM image analysis of the platinum substrate after 1 hourdeposition from 2:1 electrolyte reflected the presence of pronouncedspherical particles of 1-5 μm on the surface. An EDX full frame analysisof the SEM image confirmed that the spherical particles were magnesium.The uniform spherical magnesium deposition morphology is extremelyimportant for practical use of the electrolytes because it ensures thelack of dendrite formation in battery systems.

The electrochemical performance of the three amidomagnesium-basedelectrolytes at varying molar ratios was compared with BEC and APCelectrolytes. The electrochemical parameters obtained from the cyclicvoltammograms conducted at a 25 mVs⁻¹ scan rate are shown in Tables 1and 2 including the cyclic voltammograms of the amidomagnesium-basedelectrolytes with a direct comparison with BEC (i.e., 0.25 molarMg(AlCl₂EtBu)₂/THF) and APC (i.e., 0.4 molar 2(PhMgCl)—AlCl₃/THF)electrolyte. The 1:1 (Lewis base:Lewis acid ratio) amidomagnesium-basedelectrolyte demonstrated enhanced performance among all theamidomagnesium-based electrolytes synthesized. It showed a 1^(st) cycleCoulombic efficiency 93% and was in excellent agreement withchronopotentiometic measurements. Table 2 lists the electrochemicalparameter(s) obtained from the linear sweep voltammograms (LSV)performed at a scan rate of 1 mVs⁻¹ from open circuit potential toapproximately 4V showing the anodic oxidative stability of theamidomagnesium-based electrolytes onto platinum working electrodecompared with known anodic stability of 2.2V and 3V for BEC and APCelectrolytes, respectively. It was found that the electrochemicalstability of the amidomagnesium-based novel electrolytes was ˜2.27V,˜2.33V and ˜2.19V for 1:1, 2:1 and 3:1 electrolytes, respectively. Theelectrochemical anodic stability of the amidomagnesium-based electrolytewas similar to Aurbach's BEC electrolyte but significantly lower thanAPC electrolyte.

In order to validate the feasibility of the amidomagnesium-basedelectrolyte solutions for a magnesium battery system, Mo₆S₈ Chevrelphase a known magnesium-ion intercalation cathode was synthesized bymolten salt (MS) route found in literature and high energy mechanicalmilling (HEMM) route first time reported. The 2016-type coin cell wasconstructed using the 0.4M L⁻¹ [(CH₃)₃Si]₂NC₆H₄MgCl]—AlCl₃/THF solutionas the electrolyte (3:1 molar ratio), a Mg disc as a negative electrode,and Mo₆S₈ as a positive electrode. The 2016 coin cells were cycled at acurrent rate of C/6 (˜20 mAg⁻¹) with the discharge and charge voltagelimits of 0.5V and 1.5V versus Mg reference electrode at roomtemperature. The results showed that the 1^(st) cycle discharge andcharge capacity was ˜128 mAhg⁻¹ (100% of theoretical capacity) and ˜70mAhg⁻¹ with 1^(st) cycle Coulombic efficiency ˜54.8% for HEMM derivedMo₆S₈ cathode. The expected drop in 1^(st) cycle capacity was due topartial charge trapping which occurred in the Mo₆S₈ electrode during1^(st) cycle. Nevertheless, Mo₆S₈ electrode derived by MS and HEMMmethod was able to cycle magnesium reversibly and yield a dischargecapacity of ˜66 mAhg⁻¹ and ˜60 mAhg⁻¹ calculated based on the weight ofthe cathode's active mass with Coulombic efficiency ˜97% and ˜95%respectively, between 2^(nd) and 50^(th) cycle. Typical cyclicvoltammogram conducted between 0.5-1.5V at a scan rate of 0.1 mVs⁻¹shows cathodic and anodic peak at ˜1.0V and ˜1.3V due to formation ofMg_(x)Mo₆S₈ (0<x≤1) phase.

Conclusions

In summary, we have synthesized a ˜2.3V amidomagnesium based magnesiumelectrolyte system based on the Lewis base-acid complex via a reactionbetween organometallic Grignard reagent [(CH₃)₃Si]₂NC₆H₄MgCl] and AlCl₃in tetrahydrofuran solvent. The [(CH₃)₃Si]₂NC₆H₄MgCl]AlCl₃/THEelectrolyte solution shows excellent reversibility of Mgdeposition-dissolution (˜90% cycling efficiency for reversible magnesiumdeposition), and electrochemical anodic stability (2.3V vs. Mg referenceelectrode). In addition, the good compatibility of theamiodomagnesium-based electrolyte solution with the Mo₆S₈ intercalationcathode derived by the molten salt as well as the high energy mechanicalmilling routes confirms that the electrolyte could be practically usedin 2V rechargeable Mg battery systems.

Example XIII

High energy mechanical milling (HEMM) of a stoichiometric mixture ofmolybdenum and copper chalcogenide (CuT and CuT₂), followed by a shortthermal treatment at elevated temperature was applied to synthesizeChevrel phases (Cu₂Mo₆Z₈; Z ═S, Se), a cathode precursor for magnesiumbattery. Differential scanning calorimetry, thermo-gravimetric analyses,combined with X-rays diffraction and scanning electron microscopy wasused to evaluate the phase transformation(s) during milling and thermaltreatment. It was shown that CuS and Mo reacted at elevated temperatureand formed an intermediate ternary Chevrel phase which further reactedwith residual Mo and MoS₂ to form the desired Cu₂Mo₆S₈. Quantitative XRDanalyses showed the formation of ˜96-98% Chevrel phase as low as 30minutes time during post milling thermal treatment process.Electrochemical performance of de-cuprated Mo₆S₈ and Mo₆Se₈ phase wereevaluated by cyclic voltammetry (CV), galvanostatic cycling,electrochemical impedance spectroscopy (EIS). CV and galvanostaticcycling data of Mo₆S₈ and Mo₆Se₈ electrodes showed expectedanodic/cathodic behavior and a stable capacity after the 1^(st) cyclewith the formation of Mg_(x)Mo₆Z₈ (Z═S, Se; x=1≤x≤2). EIS at ˜0.1 Vintervals of Mo₆S₈ electrode during 1st and 2^(nd) cycle showed thatpartial Mg-ion trapping caused increased charge transfer resistance,R_(e). Carbon incorporation during milling resulted in improved capacityfade in the case of Mo₆Se₈ electrode and ˜99.93% Coulombic efficiencywas achieved. Importantly, ease of fabrication, stable capacity, highCoulombic efficiency and excellent rate retention rendered HEMM-derivedChevrel phases as suitable magnesium battery cathodes for stationaryelectrical energy storage (EES) applications.

Electrochemical performance of HEMM-derived Mo₆S₈ and Mo₆Se₈ phasedevoid of copper was evaluated in 2016 coin cell by cyclic voltammetry,and galvanostatic cycling at various C— rates. The Chevrel phase(s)exhibited competitive electrochemical results and provided a timesaving, rapid approach.

Materials Synthesis

Cu_(x)Mo₆S₈ was synthesized as follows using an HEMM approach inaccordance with certain embodiments of the invention. Stoichiometricamounts of MoS₂ (1 g, 99% Alfa Aesar), Mo (0.6 g, 99.9% Alfa Aesar), andCuS (0.4 g, 99.8% Alfa Aesar) were batched in a stainless steel vial(powder: ball ratio=1:10). The powders were mechanically milled inatmospheric condition for 30 minutes, 1 hour, 2 hours, and 3 hours in aSPEX-8000M shaker mill and subjected to XRD analysis. Cu_(x)Mo₆Se₈ wasalso synthesized using a similar approach. Stoichiometric amounts ofMoSe₂ (1.141 g, 99.9% Alfa Aesar), Mo (0.432 g, 99.9% Alfa Aesar), andCuSe (0.427 g, 99.5% Alfa Aesar) were batched in a stainless steel vial(powder: ball ratio=1:10). The powders were mechanically milled for 30minutes, 1 hour, 2 hours, and 3 hours, and subjected to XRD analysis.The milled powder was thermal treated at elevated temperature underultra-high purity Argon (UHP-Ar) atmosphere. Copper-ions weresubsequently leached out from HEMM-derived ternary Chevrel phase(s)using 6 molar hydrochloric acid solution under constant oxygen flow for8 hours in ambient atmosphere. After copper-ions were leached completelyfrom the parent phase, the acid solution containing the residue wascentrifuged, washed with de-ionized water three times and dried in anoven at ˜323K.

Materials Characterization

The thermal behavior of the milled powder was evaluated bythermo-gravimetric (TG)/differential scanning calorimetry (DSC) using aNetzsch STA 409 PC Luxx thermal analyzer unit in an ultra UHP-Aratmosphere at a heating rate of 10 K/min, up to 1273K. The milled powdersamples were isothermally heat treated at 1273K for 30 minutes underUHP-Ar atmosphere followed by X-rays diffraction (XRD) analyses. Inaddition, the mechanically milled powder was thermal treated at elevatedtemperature for different durations (30 minutes, 5 hours) under UHP-Aratmosphere. Powder X-ray diffraction (XRD) of the milled, commercial andthermally-treated powders was carried out using the Philips PW1830system employing the CuK_(α) (λ=0.15406 nm) radiation. Quantitativephase analysis was carried out using PANalytical X'Pert HighScore PlusRietveld program on the powder XRD patterns. No other attempt was madeto determine the composition obtained from the HEMM approach at hightemperature. Microstructural analyses of the milled, commercial and heattreated powders (Chevrel phases) were performed using a scanningelectron microscopy (JSM-6610, JEOL) operating at 10 kV and highresolution transmission electron microscopy-HRTEM (JEOL JEM 2000FX)operating at 200 kV. Specific surface area of the milled powder wasmeasured using the Brunauer-Emmett-Teller (BET) technique. Each samplewas vacuum degassed and then tested using a Micromeritics ASAP 2020 BETequipment.

Electrochemical Characterization

Electrode slurry was formulated by mixing 80 wt. % of active material(˜325 mesh), 10 wt. % Super-P carbon, 10 wt. % polyvinylidene fluoride(PVDF) binder with N-methylpyrrolidinone (NMP) solvent together in aglass vial with constant magnetic stirring for 24 hours. The slurryobtained was coated (50 μm thick) onto graphite foil acting as a currentcollector and dried at ˜383K overnight in a vacuum oven. Further, thedried electrodes were uniaxially pressed at ˜5 MPa to improve theparticle contacts and then circular disks (Θ=11.28 mm) were punched withan active material loading ˜1-3 mg/cm². An electrochemical test wascarried out at room temperature with 2016-type coin cells assembledinside an argon-filled MBraun Inc. glove box (<0.1 ppm each of O₂ andH₂O) employing magnesium foil as the counter and reference electrode,electrode disks as working electrode, and Celgard® separator soaked inan electrolyte solution of 0.4 molar 2(PhMgCl)—AlCl₃/tetrahydrofuran.Cyclic voltammogram (CV) was acquired using an electrochemicalworkstation (VersaSTAT 3, Princeton Applied Research) at a constantsweep rate of ˜0.001 Vs⁻¹. Galvanostatic charge-discharge cycles werecarried out at various rates ˜20-160 mAg⁻¹ within 0.5-1.5V or 0.5-1.7V,employing a short rest period between the charge/discharge cycles usinga multichannel battery testing system (Arbin BT2000 instrument).Electrochemical impedance spectroscopy (EIS) was performed to understandthe charge storage behavior in Chevrel phases. EIS was performed on theVersastat 3 over a frequency range of 0.01 Hz-100 KHz. An A.C. amplitudeof 5 mV was used and the spectra were obtained after charge/dischargecycles. The charge transfer characteristics and accompanying parameterswere analyzed by equivalent circuit modeling using Z-View (ScribnerAssociates, Inc.; version 3.3).

Results and Discussion

Synthesis of Cu_(x)Mo₆S₈ Chevrel Phase by HEMM Approach

Each as-received commercial powder as well as mechanically milledpowders was subjected to XRD analysis. The major XRD peaks fromcommercial powders can be indexed with standard Mo (ICDD number:98-006-2711; cubic; space group Im-3m, number 229), CuS (ICDD number:01-079-2321; hexagonal; space group P63/mmc, number 194), and MoS₂ (ICDDnumber: 03-065-7025; hexagonal; space group P63/mmc, number 194) phase.From the XRD pattern of mechanically milled powders, it appeared thatgradual increase in milling duration from 30 minutes to 3 hours induceda homogeneous mixture between CuS, Mo, MoS₂ phase. The Bragg reflectionof high intensity peaks from MoS₂, Mo and CuS were evident in the XRDpatterns of milled powders. The absence of any peaks besides CuS, Mo andMoS₂ suggested that mechanical milling only induced an intimate mixturebetween the constituent phases rather than forming any new phase(s).However, gradual increase in milling duration would likely induce anX-rays amorphization of the MoS₂ and CuS phase either due to formationof ultrafine particles or diffusive mixing mechanisms initiated betweenthe CuS and MoS₂ phases which would significantly decrease the peakintensity and allow peak broadening. The relative intensity of peaksfrom (002), (010), (013), (105) and (112) planes of MoS₂ and (102),(013), (006), (110) planes of CuS were significantly reduced and peakswere broadened as the milling duration increased from 30 minutes to 3hours. The Bragg reflection from (011), (002), (112), and (022) planesreflected from elemental Mo were found to be relatively intense. Thedifference in peak intensities and broadening during milling operationwas mainly due to a difference in the hardness of starting compositions,where ductile and hard Mo metal powder repeatedly got fractured and coldwelded whereas brittle and soft MoS₂, and CuS ceramic phase gotfragmented and embedded within the ductile Mo matrix during millingoperation.

SEM images of the commercial powder used during milling showed uniquemorphology of the particles before milling. CuS particles were ofirregular shape and agglomerated, whereas MoS₂ were large and flaky, andMo particles were round and globular. Following 30 minutes of mechanicalmilling, the morphology of the particles changed completely andirregular shape particles were formed. SEM image of the 30 minute-milledpowder showed formation of agglomerated irregular shaped particlesmechanically bonded together. Quantitative elemental composition of themilled powder obtained by EDX analyses confirmed that measured atomicpercent of each element (Cu, Mo, and S) was close to the stoichiometricbatch composition. The elemental X-ray mapping of Cu, Mo, and S atoms inthe milled powder showed homogeneous distribution of Cu, Mo, and Swithin the agglomerated particles without segregation on any specificsite. Importantly, the absence of any oxides of copper and molybdenum upto 3 hours of milling was evident from the XRD patterns which suggestedthat milling between CuS, MoS₂, and Mo in atmospheric conditions doesnot oxidize or contaminate the milled composition. However, mechanicalmilling did not form the desired ternary CP (since Chevrel phaseformation is a thermally activated metastable process and requiresthermal treatment at elevated temperature under inert atmosphere to formthe desired phase). The Brunauer-Emmett-Teller (BET) surface areameasurement by nitrogen adsorption/desorption technique of the milledpowder was ˜12.33 m²/g, 14.61 m²/g, 7.72 m²/g, and 9.65 m²/g, for 30minute-, 1 hour-, 2 hours-, and 3 hours-milled powder sample,respectively. From the surface area measurement it appeared thatmechanical milling beyond 1 hour had no effect on reduction of surfacearea of milled particles. However, to complete the formation ofhomogeneous mixture between the constituents the mechanical milling wascontinued up to 3 hours.

To assess phase formation during thermal treatment of the mechanicallymilled powder, selected milled samples were subjected to thermogravimetric-differential scanning calorimetry (TG-DSC) under UHP-Aratmosphere up to 1273K at a constant heating rate of 10K/min. The TG-DSCtrace showed a continuous exothermic behavior of the milled powdersamples. The 30 minutes-milled sample showed a broad exothermic peakbetween 600-1260K associated with an enthalpy of formation ˜775 J/g(˜743 kJmol⁻¹) approximated to the calculated enthalpy of formation(˜873 kJmol-1) for Cu₂Mo₆S₈ at 1000K. DSC traces of 1 hour- and 3hour-milled powder samples showed a continuous exothermic behaviorbeyond 600K with no apparent exotherms up to 1273K. However, thermogravimetric (TG) curves exhibited˜4% weight loss between ˜525K to ˜630Kfor each sample. The 4% weight loss associated with TG analysis wasaccompanied by heat evolution in the DSC curves which suggested that anexothermic process was initiated. To assess the exothermic reactionprocesses evolved with DSC scan of milled powder sample, each of theconstituents (CuS, Mo and MoS₂ powder) used during milling was subjectedto TG-DSC under identical conditions. The thermo gravimetric trace of Moand MoS₂ commercial powder showed no appreciable weight change. However,CuS showed ˜16% weight loss when heated to ˜745K that was accompanied byan endothermic reaction (enthalpy of formation: ˜206 J/g or 19.7 kJ/mol)between ˜686K to ˜773K with the peak endotherm at ˜745K. Withoutintending to be bound by any particular theory, it is believed that theendothermic peak was due to formation of Cu₂S and sulfur-rich liquidphase from CuS since the thermodynamic measurement of enthalpy offormation for peritectic decomposition of CuS to Cu₂S and S-rich liquidwas ˜17.87 kJ/mol. The formation of Cu₂S from CuS resulted in a 16%weight difference according to the following formula:—2 CuS(2×95.61)═Cu₂S (159.16)+S (32.06). The ˜4% weight loss (¼th of ˜16%)observed was related to the phase transformation of CuS to Cu₂S andsulfur-rich liquid phase and was in agreement with the followingformula:—2 CuS+3Mo+3MoS₂═Cu₂Mo₆S₈. It is believed that the sulfur-richliquid reacted immediately with elemental Mo and formed the MoS₂ phasewhich subsequently reacted with Cu₂S, unreacted Mo and MoS₂ and formedthe desired ternary copper CP according to the followingformula—Cu₂S+3MoS₂+xMoS₂+(3-x)Mo+(1-2×)S═Cu₂Mo₆S₈.

To assess the details of phase formation during thermal treatment,equimolar amounts of Mo (˜1 g) and CuS(˜1 g) were mechanically milledfor 30 minutes and subjected to TG-DSC. The TG-DSC trace of CuS—Mopowder exhibited three regions, a small endothermic hump at ˜714K (dueto peritectic reaction and melting of CuS to Cu₂S and S) associated with˜8% weight loss, a plateau between the temperature interval of714K-800K, and a continuous exothermic region beyond 800K up to 1273K.It appeared that the peritectic reaction at ˜714K occurred slightlyearlier than expected likely due to mechanical milling having decreasedthe particle size and collapsed the hexagonal crystal structure of CuS.The XRD pattern of the mechanically milled powder showed the presence ofMo and CuS phase. XRD analysis was performed on the remnant powder afterthe TG-DSC test was completed. The XRD showed the formation ofCu_(1.83)Mo₃S₄ phase (ICDD: 98-000-5103), along with elemental Mo andCu. It is believed that CuS dissociated into Cu₂S and elemental sulfurand further reacted with Mo to form the Cu_(1.83)Mo₃S₄ Chevrel phaseaccording to the following reactions:

CuS=Cu₂S+Mo (˜714 to ˜745K)

2 Cu₂S+2S+4Mo═Cu_(1.83)Mo₃S₄+2.17 Cu+Mo (between 800K-1273K).

Thermal treatment of CuS+Mo powder was carried out inside a tube furnaceat two different temperatures of ˜773K (above the peritectic temperatureof CuS) and ˜1273K at a heating rate of 10K/min and allowed the powderto furnace cool under UHP-Ar atmosphere. The XRD pattern of the samplethermally treated to ˜773K showed the presence of Cu₂S and elemental Mo,whereas, the sample heated to ˜1273K showed the formation ofCu_(1.83)Mo₃S₄, Cu and Mo peaks. Similar study between equimolar Mo(0.75 g) and MoS₂ (1.25 g) showed the small endotherm 711K associatedwith ˜8% weight loss due to the peritectic reaction melting of CuS inthe TG-DSC trace. However, the CuS+MoS₂ milled powder when heated to˜773K and ˜1273K respectively, showed no traces of Chevrel phaseformation confirmed by XRD analyses, instead Cu₂S and MoS₂ phase wereintact. The above evaluations suggested that the presence of CuS and Moin the starting composition was essential for the formation of ternarycopper CP. One possible reaction scheme for the formation of Cu₂Mo₆S₈ternary CP at elevated temperature may be Cu_(1.83)Mo₃S₄+2.17Cu+Mo+2Mo+6MoS₂=2 Cu₂Mo₆S₈.

To assess whether ternary CP can be formed during thermal treatment, aXRD analysis of the remnant powder was conducted following completion ofthe TG-DSC on 2 CuS+3Mo+3MoS₂ samples milled for 30 minutes and 1 hour.The alumina crucible used during the TG-DSC test was carefully openedupon cooling to room temperature and the sample (˜10 mg) was evenlyspread onto a glass slide and a XRD analysis between 10-90° 20 value wasperformed. Unexpectedly, the XRD data of a 1 hour milled sample heatedto 1273K at a constant heating rate of ˜10K/min showed the formation ofCu₂Mo₆S₈ phase along with unreacted MoS₂, Mo, and MoO₂ phase asimpurities. It is believed that Cu₂Mo₆S₈ was formed before thetemperature was reached to 1273K. However, incompleteformation/transformation of Cu₂Mo₆S₈ phase was mainly due toinsufficient time to allow completion of the solid state diffusivereaction to occur between the constituents. A similar TG-DSC evaluationwas repeated up to 1273K with an additional 30 minute-holding time at1273K with 1 hour- and 3 hour-milled powders. The XRD analysis performedon 1 hour- and 3 hour-mechanically milled powder following TG-DSC testshowed the formation of Cu₂Mo₆S₈ phase with minor unreacted MoS₂ and Mo.According to the Cu—Mo—S ternary phase diagram at 1273K, Cu_(x)Mo₆S₈phase could co-exist with MoS₂ and metallic Mo and appear as minorimpurities during high temperature synthesis of the ternary Chevrelphase, M_(x)Mo₆S₈ (M=Cu, Sn). The above results were encouraging andindicated that ternary copper CP (Cu₂Mo₆S₈) could be formed using CuS,Mo, and MoS₂ powder at 30 minute-dwell time at 1273K peak temperature.The magnified portion between 20 values of 12-16° of the XRD dataclearly showed that 30 minute-dwell time at 1273K had a pronouncedeffect on Cu₂Mo₆S₈ formation, as the high intensity peak from (002)plane of MoS₂ was significantly reduced at the expense of peak intensityfrom (101) plane of Cu₂Mo₆S₈ phase. An ˜2 g powder sample of 30 minute-and 1 hour-milled powder was placed on a corundum crucible and thermallytreated inside a tubular furnace with a heating rate of 10K/min at twodifferent temperatures of 1123K and 1273K with a 30 minute-dwell time atpeak temperature followed by cooling to room temperature at a constantcooling rate of 10K/min (the above experiments were done under UHP-Ar,gas flow rate ˜100 standard cubic centimeter/min). The heat-treatedpowder analyzed by XRD showed the formation of ternary copperCP—Cu₂Mo₆S₈. The 30 minute-mechanically milled powder sample thermallytreated to 1123K with 30 minute-dwell time showed the formation ofCu₂Mo₆S₈ with slight presence of unreacted MoS₂ and Mo powder. However,the 30 minute- and 1 hour-milled powder samples when heated to 1273Kwith 30 minute-dwell time at the peak temperature, the MoS₂ peak wascompletely vanished and fully crystalline Cu₂Mo₆S₈ phase was formed. Themagnified portion between 12-16° of the XRD patterns depicted the highintensity peak from (101) plane of Cu₂Mo₆S₈ phase matched completelywith the standard Cu₂Mo₆S₈ database (ICDD number: 00-047-1519).Secondary electron SEM image of the Cu₂Mo₆S₈ phase synthesized at 1123Kand 1273K with 30 minute-dwell time at peak temperature showed theformation of sub-micron to micrometer size irregular shaped particles.The composition, obtained by EDX quantitative analyses, of the copperChevrel phase formed at 1123K and 1273K, respectively, was in goodagreement with TG-DSC/XRD data and suggested that rapid synthesis ofCu₂Mo₆S₈ by HEMM was possible and could be achieved in a time period aslow as at 30 minutes. Lattice parameter(s) calculated using theleast-square method of the experimental XRD data was in good agreementwith the standard Cu₂Mo₆S₈ (ICDD number: 00-047-1519; a=0.9584 nm,c=1.025 nm, unit cell volume=815.36×10³ nm³) unit cell parameters andpresented in Table 1.

Phase Formation and Electrochemical Activity of HEMM Derived De-CupratedMo₆S₈

To obtain the de-cuprated Mo₆S₈ phase, Cu₂Mo₆S₈ phase formed at 1123Kand 1273K was treated with 6 molar hydrochloric acid solution withconstant O₂ bubble for 7 hours in a round bottom flask. In order toconfirm the formation of Mo₆S₈ after complete copper leaching, XRDanalysis was performed. All the Braggs peaks in the XRD patterns matchedwith the standard Mo₆S₈ (ICDD: 98-005-708) pattern which suggestedcomplete copper removal was achieved. EDX full frame analyses alsoconfirmed that hydrochloric acid-treated samples were devoid of copper.High resolution SEM image of the respective Mo₆S₈ phase showedagglomerated particles of submicron to micrometer size. Electrochemicalperformance of the 80 wt. % Mo₆S₈: 10 wt. % Super-P: 10 wt. % PVDFcomposite electrode was evaluated in a 2016 coin cell. The galvanostaticcycling conducted at ambient condition between 0.5-1.7V with a currentof 20 mAg⁻¹ (˜C/6 rate) showed 1^(st) cycle discharge capacity of ˜101mAhg⁻¹ and ˜100 mAhg⁻¹ for de-cuprated Cu₂ CP formed at ˜1123K and˜1273K, respectively. A steady and expected discharge capacity of ˜68mAg⁻¹ and ˜70 mAhg⁻¹ was achieved from 2^(nd) to 100^(th) cycle due topartial Mg-ion trapping within Mo₆S₈ structure and associated 30% dropin capacity in the 1st cycle. The cyclic voltammogram of the Mo₆S₈electrode showed expected signature of two anodic peaks duringmagnesiation and a cathodic peak during de-magnesiation, respectively.

To achieve the fully crystalline phase pure Cu₂Mo₆S₈ ternary CP, dwelltime at peak temperature of ˜1273K was increased to 5 hours. The XRDpattern of 3 hour-mechanically milled powder heat treated at 1273K withthe holding time of 5 hours at peak temperature confirmed the completeformation of Cu₂Mo₆S₈. De-cuprated Mo₆S₈ was obtained after completecopper removal using a 6 molar HCl solution with constant oxygenbubbling for 7 hours. The lattice parameter(s) calculated usingleast-square method of Cu₂Mo₆S₈ phase formed after 3 hour-mechanicallymilled powder thermally treated at ˜1273K with 5 hours dwell time atpeak temperature are shown in Table 4 and are in good agreement withstandard unit cell parameter(s).

TABLE 4 Calculated lattice parameter(s) of Cu_(x)Mo₆S₈ phase.Quantitative Lattice parameter(s) of Phase formation Condition Cu₂Mo₆S₈(%) 1 h mechanically a_(H): 0.963138 nm, c_(H): 0.3% Mo + milled powderthermally 1.021042 nm 99.7% Cu₂Mo₆S₈ treated to 1273 K with a_(R):0.651957 nm, α_(R): 30 min hold at 1273 K 95.233° unit cell volume =820.2588 × 10⁻³ nm³ 30 min mechanically a_(H): 0.962119 nm, c_(H): 0.3%Mo + milled powder thermally 1.021095 nm 99.7% Cu₂Mo₆S₈ treated to 1273K with a_(R): 0.651464 nm, α_(R): 30 min hold at 1273 K 95.1953° unitcell volume = 818.567 × 10⁻³ nm³ 30 min mechanically a_(H): 0.959014 nm,c_(H): 1.4% MoS₂ + milled powder thermally 1.022991 nm 0.3% Mo + treatedto 1123 K with a_(R): 0.650268 nm, α_(R): 98.3% Cu₂Mo₆S₈ 30 min hold at1123 K 95.0207° unit cell volume = 814.8032 × 10⁻³ nm³ 3 h mechanicallya_(H): 0.961938 nm, c_(H): 100% Cu₂Mo₆S₈ milled powder thermally1.021293 nm treated to 1273 K with a_(R): 0.65141 nm, α_(R): 5 h hold at1273 K 95.1822° unit cell volume = 818.4188 × 10⁻³ nm³

Whereas, lattice parameter(s) of de-cuprated Mo₆S₈ calculated using theleast-square method (a=0.919233 nm, c=1.087942 nm, unit cellvolume=796.1371×10⁻³ nm³) were found to be in excellent agreement withthe standard Mo₆S₈ (ICDD number: 98-005-1708; a=0.92 nm, c=1.088 nm,unit cell volume=797.51×10⁻³ m³) unit cell parameters showed thatcomplete removal of copper was achieved. The SEM image of Cu₂Mo₆S₈ andde-cuprated Mo₆S₈ showed irregular shape submicron size particles thatwere formed in the case of Cu₂Mo₆S₈ phase, which retained theirmorphology in the de-cuprated Mo₆S₈ phase as well. The HRTEM latticefringe spacing of Cu₂Mo₆S₈ and Mo₆S₈ phase was calculated as ˜0.645 nmand ˜0.479 nm which corresponded to the interplanar d-spacing of (101)and (102) planes for Cu₂Mo₆S₈ and ˜0.641 nm for Mo₆S₈ corresponding tothe interplanar d-spacing of (101) plane (hexagonal crystal system:space group R-3). The de-cuprated Mo₆S₈ phase was then tested as amagnesium battery cathode in a 2016 coin cell setup using2(PhMgCl)—AlCl₃/tetrahydrofuran electrolyte and polished Mg foil as theanode. The electrochemical performance of Mo₆S₈ composite electrodeshowed outstanding performance where Mg-ions could be inserted/extractedreversibly from the de-cuprated Mo₆S₈ phase. The cyclic voltammogramacquired at a sweep rate of 0.1 mVs⁻¹ between 0.5-1.75V versus Mg/Mg²⁺showed typical reversible magnesiation/demagnesiation phenomena of Mo₆S₈electrode where magnesiation and demagnesiation occurred at ˜1V and˜1.28V, respectively. The galvanostatic cycling of Mo₆S₈ electrodeshowed 1^(st) cycle discharge (magnesiation) and charge (demagnesiation)capacity of ˜92 mAhg⁻¹ (˜72% of theoretical capacity 128 mAhg⁻¹) and ˜57mAhg⁻¹ with Coulombic efficiency ˜62% when cycled at a current densityof 20 mAg⁻¹. Between the 2^(n)d and 107^(th) cycle a stable dischargeand charge capacity of ˜62 mAg⁻¹ and 67 mAg⁻¹ was observed with 95.6%Coulombic efficiency. The expected drop in capacity from 2^(nd) cycleonwards was due to one Mg-ion being trapped which occurred within theMo₆S₈ framework at the 1^(st) cycle resulting in a capacity loss fromthe theoretical value (˜128 mAhg⁻¹). Capacity versus voltage profile of25^(th), 50^(th), and 100^(th) cycle exhibited magnesiation anddemagnesiation reaction plateaus at ˜1.07V and ˜1.2V and was inagreement with a CV curve and those reported in the literature. Rateretention of Mo₆S₈ electrode was also excellent, at C/24, C/12, C/6,C/4, C/3, C/2, 1 C (1 C˜120 mAg⁻¹) and 1.5 C current rate a dischargecapacity of ˜79 mAhg⁻¹, ˜74 mAhg⁻¹, ˜70 mAg⁻¹, ˜65 mAhg⁻¹, ˜62 mAhg⁻¹,˜59 mAhg⁻¹, ˜56 mAhg⁻¹, ˜51 mAhg⁻¹ and ˜49 mAhg⁻¹ was achieved with aCoulombic efficiency ˜96.8%, ˜94.2%, ˜97.1%, ˜99.2%, ˜99.9%, ˜99.9%,˜99.7%, ˜99.9%, and ˜99.2%, respectively. In addition, long cyclestability of Mo₆S₈ electrode was tested in a 2016 coin cell setup andshowed remarkable performance and durability. The 2016 coin cell wascycled continuously for 48 days and was able to deliver a specificcapacity ˜70 mAhg⁻¹ with a Coulombic efficiency ˜99% at 330^(th) cycle.These attributes support the HEMM approach for rapid synthesis ofCu₂Mo₆S₈ Chevrel phase, and as an alternative synthesis route forternary CP.

Synthesis of Cu_(x)Mo₆Se₈ Chevrel Phase by HEMM Approach

It was demonstrated that Cu₂Mo₆S₈ could be synthesized by HEMM approachand its electrochemical performance in a magnesium battery was deemedcompetitive with electrochemical data for Chevrel phases made by knownsynthesis methods. This similar HEMM approach was employed for thesynthesis of ternary Chevrel phase —Cu₂Mo₆Se₈ using CuSe, Mo, and MoSe₂as starting powders. XRD analyses showed powder XRD patterns collectedof 30 minutes, 1 hour, 2 hours, and 3 hours of mechanically milledpowder of 2 CuSe+3MoSe₂+3Mo nominal composition. The XRD patterns fromthe commercial powders were also acquired and compared. The major XRDpeaks of commercial powders were indexed with Mo, MoSe₂ (ICDD number:03-065-7025; hexagonal; space group P63/mmc, number 194), and CuSe (ICDDnumber: 98-007-1382; hexagonal; space group P63/mmc, number 194) phasealong with minor Cu₃Se₂ (ICDD number: 98-000-6312) as impurities. Agradual increase in milling duration from 30 minutes to 3 hours likelyinduced a homogeneous and intricate mechanical mixture obtained betweenCuSe, Mo, MoSe₂ phases. The XRD patterns showed the Bragg reflections ofhigh intensity peaks from MoSe₂, Mo are present but the CuSe peakscompletely vanished after 2 hours of milling. Importantly, the absenceof oxides of copper and molybdenum during milling operation indicatedthat mechanical milling between CuSe, MoSe₂, and Mo phase was alsoconducive for the direct synthesis of the Cu₂Mo₆Se phase. SEM images ofthe CuSe, and MoSe₂ commercial powder used during milling showed uniquemorphologies of the particles before milling. CuSe particles wereplate-like, whereas MoSe₂ particles were large and flaky. However, upon30 minutes of milling the morphology of the particles changed completelyand correspondingly irregular shaped particles were formed. SEM image ofthe 30 minutes milled powder showed the formation of agglomeratedirregular shape particles. EDX full frame analysis confirmed that thestoichiometry of the batch composition was maintained. Elemental X-raymapping analysis confirmed that the distribution of Cu, Mo, and Se atomsin the milled powder were homogeneous throughout the microstructure.However, it is to be noted that direct mechanical milling did not formthe desired ternary CP and thermal treatment at elevated temperatureunder inert atmosphere was required. The BET surface area of the 30minutes, 1 hour, 2 hours, and 3 hours milled powder was ˜5.82 m²/g,˜3.87 m²/g, ˜2.09 m²/g, and ˜2.29 m²/g, respectively. The BET surfacearea measurement showed that 30 minutes of milling generated fineparticles and further milling had no pronounced effect on the particlesize. However, the milling duration was continued up to 3 hours tocomplete the formation of a homogeneous intricate mixture of thedifferent reaction constituents.

The thermal behavior of the 30 minutes, 1 hour, 2 hours and 3 hoursmechanically milled powder was evaluated by thermogravimetric-differential scanning calorimetry (TG-DSC). The samples wereheated under UHP-Ar atmosphere up to ˜1273K at a constant heating rateof 10K/min. DSC trace of 30 minutes sample showed a continuousexothermic behavior up to ˜624K followed by a plateau up to ˜745K. Thethermo gravimetric (TG) curve exhibited a ˜7% weight loss within theabove temperature range. However, there was no further weight lossbeyond ˜624K and a continuous exothermic behavior was observed up to˜1273K. To assess the exothermic/endothermic behavior evolved withTG-DSC scan of milled powder sample, each constituent used duringmilling was subjected to TG-DSC under identical conditions. The TG-DSCtrace of commercial CuSe, Mo, and MoSe₂ powder from room temperature to˜1273K showed no appreciable weight change of MoSe₂ commercial powder.However, CuSe showed ˜23% weight loss between 646K-860K reflected in theTG curve, that was accompanied by two endothermic reaction peaks at˜658.8K (enthalpy: 68.45 J/g or 9.75 kJ/mol) and at ˜798K (enthalpy:10.83 J/g or 1.54 kJ/mol) in the DSC trace. The endothermic peakobserved at ˜658.8K was due to incongruent melting of CuSe and formationof Cu₂Se and Se-rich liquid and the second endothermic peak observed at˜798K was due to eutectic melting of Cu₂Se and sulfur-rich phaseaccording to the copper-selenium binary phase diagram. Previous studiesshowed that enthalpy of formation for peritectic decomposition of CuSeto Cu₂Se and Se-rich liquid was ˜11.8±0.03 kJ/mol and was in agreementwith experimental data. The formation of Cu₂Se and Se-rich liquid fromCuSe at ˜658.8K observed in the DSC curve resulted in a 27.7% weightloss according to the following formula:—2 CuSe (2×142.51)═Cu₂Se(206.05)+Se (78.96). Thus, the ˜7% weight loss (¼th of ˜27.7%) wasrelated to the phase transformation of CuSe to Cu₂Se phase and was inexcellent agreement with the following formula:—2CuSe+3Mo+3MoSe₂═Cu₂Mo₆Se. It was believed that Se-rich liquid reactedimmediately with Mo powder and formed the MoSe₂ phase and further, theremaining constituents (Cu₂Se, Mo and MoSe₂) reacted with each other andformed the desired Cu₂Mo₆Se₈ phase.

Cu₂Se+3MoSe₂+xMoSe₂+(3-x)Mo+(1-2×)Se═Cu₂Mo₆Se₈

The thermal treatment of milled powder at elevated temperature wasimportant to initiate liquid-phase sintering between the constituents.To assess the phase formation during heat treatment, ˜2 g of 1 hourmechanically milled powder sample of nominal composition 2CuSe+3Mo+3MoSe₂ was heated inside a tube furnace with a heating rate of10K/min at various temperatures followed by cooling to room temperatureat a constant cooling rate of 10K/min (these experiments were done underUHP-Ar atmosphere with gas flow rate ˜100 standard cubiccentimeter/min). The XRD pattern of the sample heated to ˜733K could beindexed to Mo, MoSe₂, Cu₂Se and MoO₂ phases. The presence of (11-1) MoO₂peaks at 26.0° 20 value was likely as a result of the oxidation of Mo toMoO₂ during thermal treatment due to the unavoidable presence of traceamount of oxygen inside the furnace. In addition, Cu₂Se phase [presenceof (111) peak at 26.7° 2θ value] was evident instead of CuSe in the XRDpattern due to incongruent melting of CuSe to Cu₂Se and Se-rich liquidat ˜659K according to the TG-DSC analysis. However, there was no Bragg'sreflection observed from elemental Se in the XRD pattern which suggestedthat Se-rich liquid immediately reacted with Mo and formed the MoSe₂phase. This is supported by the TG-DSC analysis of 1 hour milled powderas an exothermic peak is observed at ˜735K due to formation of MoSe₂phase. MoSe₂ could be formed at temperatures as low as ˜650K and isstable over wide temperature range between ˜273K to ˜1473K according tothe Mo—Se binary phase diagram. The XRD patterns of the sample heated to˜925K and ˜000K showed similar XRD peaks with the sample heated to ˜733Kwhich corroborated that higher temperature beyond ˜000K is required forCu_(x)Mo₆Se₈ phase to be formed. The powder sample when heated to ˜1089Kshowed the formation of Cu₂Mo₆Se₈ phase with the presence of elementalMo, MoSe₂, and MoO₂ as impurities. The absence of high intensity peakfrom (111) plane of Cu₂Se phase at 26.7° 2θ value in the XRD patternexcept for the ˜733K sample suggested that Cu₂Se started to disappearabove 800K due to eutectic melting of Cu₂Se phase which likely reactedwith Mo and MoSe₂ phase and nucleated the desired ternary selenium CPphase analogous to the case observed with sulfur CP (shown above). Asexpected, the sample when heated to ˜1151K also showed the formation ofCu₂Mo₆Se₈ phase. However, phase-pure Cu_(x)Mo₆Se₈ was not detected inthe XRD patterns when the sample was heated to ˜1089K or ˜1151K due toinsufficient dwell time at the respective temperature needed to allowfor complete transformation to occur. Intrigued by the aboveobservation, ˜2 g sample of 30 minutes, 1 hour, 2 hours, and 3 hours ofmechanically milled powder was placed on a corundum crucible andthermally treated at ˜1151K with 30 minute dwell time at the peaktemperature inside a tube furnace under argon atmosphere. The powder XRDanalysis of the heat-treated powder confirmed the formation of ternarycopper CP—Cu₂Mo₆Se₈ with slight presence of unreacted Mo, MoO₂ and MoSe₂phase. Full frame quantitative EDX analyses of the low magnification(1000×) SEM images also proved the presence of Cu, Mo and Se atoms inthe ternary CuMo₆Se₈ phase formed at 1151K with 30 minute dwell time aswell as in the hydrochloric acid treated de-cuprated Mo₆Se₈ phase. Theabove data was encouraging and in good agreement with TG-DSC, andsuggested that synthesis of fully crystalline Cu₂Mo₆Se₈ phase waspossible by the HEMM approach in a time duration as low as 30 minutes at˜1151K. Lattice parameter(s) of Cu₂Mo₆Se₈ phase were calculated usingthe least-square method of the experimental data and are presented inTable 5. The excellent agreement of lattice parameters with the standardunit cell parameter(s) of Cu₂Mo₆Se₈ reported in the literature suggestedthat the HEMM approach could be applied successfully for the synthesisof selenium CP with minimal thermal treatment required at ˜1151K underargon atmosphere.

TABLE 5 Calculated lattice parameter(s) of Cu_(x)Mo₆Se₈ phase.Quantitative Lattice parameter(s) of Phase formation Condition Cu₂Mo₆Se₈(%) 1 h mechanically a_(H): 0.999259 nm, c_(H): 0.2% Mo + milled powder1.072559 nm 1.8% MoO₂ + thermally treated a_(R): 0.678719 nm, α_(R):1.7% MoSe₂ + to 1089 K 94.8065° 96.3% Cu₂Mo₆Se₈ unit cell volume =927.4872 × 10⁻³ nm³ 1 h mechanically a_(H): 0.997191 nm, c_(H): 0.3%Mo + milled powder 1.07323 nm 1.8% MoO₂ + thermally treated a_(R):0.677823 nm, α_(R): 1.7% MoSe₂ + to 1151 K 94.7132° 96.2% Cu₂Mo₆Se₈ unitcell volume = 924.2306 × 10⁻³ nm³ 30 min mechanically a_(H): 0.972965nm, c_(H): 0.2% Mo + milled powder 1.092789 nm 1.9% MoO₂ + thermallytreated a_(R): 0.669508 nm, α_(R): 1.7% MoSe₂ + to 1151 K with 93.2087°96.2% Cu₂Mo₆Se₈ 30 min hold at unit cell volume = 1151 K 895.9042 × 10⁻³nm³ 3 h mechanically a_(H): 1.002506 nm, c_(H): 1.8% MoO₂ + milledpowder 1.07251 nm 1.7% MoSe₂ + thermally treated a_(R): 0.680305 nm,α_(R): 96.5% Cu₂Mo₆Se₈ to 1273 K with 94.9202° 5 h hold at unit cellvolume = 1273 K 933.4816 × 10⁻³ nm³

Phase Formation and Electrochemical Activity of HEMM Derived De-CupratedMo₆Se₈

The de-cuprated Mo₆Se₈ phase was obtained by completely leaching outcopper from Cu₂Mo₆Se₈ phase formed at 1151K with 30 minutes dwell timeat the peak temperature. The XRD patterns analysis confirmed theformation of Mo₆Se₈ phase along with minor MoO₂ and unreacted MoSe₂phase. The major Bragg peaks matched with the standard Mo₆Se₈ pattern(ICDD: 01-085-0455) suggested copper was completely removed. SEM imageof the Mo₆Se₈ phase showed particles of submicron to micrometer size.Electrochemical performance of the 80 wt. % Mo₆Se₈: 10 wt. % Super-P: 10wt. % PVDF composite electrode was evaluated in a 2016 coin cell. Theelectrochemical test showed the 1st cycle discharge (magnesiation) andcharge (de-magnesiation) capacity was ˜58.5 mAhg⁻¹ and ˜64 mAhg⁻¹ withCoulombic efficiency ˜91.5%. In the 2nd cycle the discharge and chargecapacity were ˜59.4 mAhg⁻¹ and ˜70.5 mAhg⁻¹ and in the 3rd cycle, thedischarge and charge capacity were ˜63.8 mAhg⁻¹ and ˜75.5 mAhg⁻¹,respectively. It is apparent from the voltage profile that a singlevoltage plateau was observed during the 1st cycle discharge (˜0.85V) andcharge (˜1.17V) whereas 2-steps magnesiation at ˜1.05V and ˜0.97V andde-magnesiation reactions at ˜1.1V and ˜1.17V were evident in the 2ndand 3rd cycle. The detailed subtle differences in the charge-storagemechanisms into Mo₆Se₈ phase from the 1st cycle and the subsequentcycles are discussed later herein. However, the competitiveelectrochemical data of de-cuprated Mo₆Se₈ phase obtained from Cu₂Mo₆Se₈phase formed at 30 minutes suggested the ease of synthesis of thecorresponding selenium CP.

The fully crystalline, Cu₂Mo₆Se₈ ternary CP was also obtained uponheating the 3 hours mechanically milled sample at ˜1273K with additional30 minutes dwell time at peak temperature. The XRD pattern of 3 hoursmechanically milled powder thermally treated at ˜1273K with 5 hoursdwell time at peak temperature showed the formation of Cu₂Mo₆Se₈ phasealong with MoSe₂ and MoO₂ as impurities (Table 5). De-cuprated Mo₆Se₈was obtained by complete leaching of copper using a 6 molar HCl solutionunder oxygen bubbling for 7 hours. The lattice parameter(s) of Cu₂Mo₆Se₈was calculated using least-square method and are shown in Table 5. Thelattice parameter(s) of Cu₂Mo₆Se₈ was in good agreement with standardunit cell parameter(s). Whereas, lattice parameter(s) of de-cupratedMo₆Se₈ (a=0.955858 nm, c=1.116432 nm, unit cell volume=883.3835×10-3nm3) was found to be in excellent agreement with the standard Mo₆Se₈(ICDD number: 01-085-0455; a=0.95488 nm, c=1.12095 nm, unit cellvolume=884.40×10-3 nm3) unit cell parameters suggesting that copper wascompletely removed by HCl treatment from the Cu₂Mo₆Se₈ phase. The SEMimage of Cu₂Mo₆Se₈ and de-cuprated Mo₆Se₈ showed irregular shapedsubmicron size particles that were formed in the case of Cu₂Mo₆Se₈ phasewhich retained its morphology completely in the de-cuprated Mo₆Se phase.The HRTEM lattice fringe spacing was calculated as ˜0.668 nm and ˜0.35nm corresponded to the interplanar d-spacing of (101) and (202) planesfor Cu₂Mo₆Se₈ and ˜0.666 nm for Mo₆Se₈ corresponding to the interplanard-spacing of (101) plane (hexagonal crystal system: space group R-3).

The composite electrode consisted of 80 wt. % Mo₆Se₈ phase (˜325 mech)with 10 wt. % Super-P carbon additive and 10 wt. % PVDF binder. It wasthen tested in a 2016 coin cell setup as a cathode and polished Mg foilas anode separated by a Celgard® separator soaked in2(PhMgCl)—AlCl3/tetrahydrofuran electrolyte. It showed goodelectrochemical performance. The cyclic voltammogram, acquired at asweep rate of 0.1 mVs-1 between 0.5-1.5V versus Mg/Mg²⁺ couple, showed atypical two-step reversible magnesiation/demagnesiation phenomena wheremagnesiation occurred at ˜1.04V, and ˜0.96V and demagnesiation at 1.11V,and A1.18V, respectively. The anodic/cathodic peaks matched well withthe theoretical calculation of the magnesiation/demagnesiation voltage.It was noted that magnesium intercalation/de-intercalation phenomenafrom Mo₆Se₈ phase was completely reversible and no partial Mg-iontrapping occurred as in the Mg_(x)Mo₆S₈ phase due to the higherpolarizability of the selenium anionic framework. The electrochemicalperformance of Mo₆Se₈ composite electrode as well as the Mo₆Se₈-graphite(7:3 weight ratio) in situ composite electrode were compared when testedat a current rate of 20 mAg⁻¹. The galvanostatic cycling of Mo₆Secomposite electrode showed a 1^(st) cycle discharge (magnesiation) andcharge (demagnesiation) capacity of ˜83.5 mAhg-1 (˜95% of theoreticalcapacity—88 mAhg⁻¹) and ˜76.3 mAhg⁻¹ with Coulombic efficiency ˜91.4%.Between the 2^(n)d and 100 cycle, an average discharge and chargecapacity of ˜71.2 mAhg⁻¹ and 74.2 mAhg⁻¹ was observed with ˜96%Coulombic efficiency. The gradual fade in capacity from the 2^(nd) to100 cycle accompanied by ˜4% irreversible loss per cycle was due to achange in the crystal structure of magnesiated Mo₆Se₈ from rhombohedral(R3) to triclinic (P1) phase and corresponding loss in electronic andionic conductivity. The Mg₁Mo₆Se₈ and Mg₂Mo₆Se₈ crystallized into thetriclinic form at room temperature by cation displacement or due tocation ordering of crystal structure which caused expansion of the unitcell and resulted in loss of electrical contact of the active mass withcurrent collector. The calculated molar volume of Mo₆Se (R0), Mg₁Mo₆Se₈(T1) and Mg₂Mo₆Se₈ (T2) phase were ˜88.78 cm³/mol, ˜184.38 cm³/mol, and˜195.63 cm³/mol, respectively. The R0 to T1 transformation wasassociated with ˜107% cell volume expansion which may cause loss ofelectrical contact between the active mass and current collector as wellas inherently poor electronic and ionic conductivity of the triclinicChevrel phase resulting in ˜4% capacity fade. In the Mg₂Mo₆Se₈ phase,both the Mg-ions were located in the tetrahedral sites of outer ring ofcavity 2 and formed a new type of cationic arrangements. The structuralrearrangement occurred where four cation sites (2 tetrahedra and twosquare-pyramids) in cavity 1 for Mg₁Mo₆Se₈ degenerated into sixtetrahedral sites. Thus, it appeared that the triclinic phase (T1 andT2) is suitable for Mg-ion diffusion and is free from any chargetrapping although the loss in capacity gradually is due to poorionic/electronic conductivity of the magnesiated Mo₆Se₈ phase.

To counteract the loss in electronic/ionic conductivity and associatedcapacity fade, a bottom-up approach was employed. First, Mo₆Se₈particles embedded within the graphitic matrix were obtained usingsynthetic graphite during milling operation. 70 wt % of (2CuSe+3Mo+3MoSe₂) powder with 30 wt. % synthetic graphite (Aldrich, 1-2μm) was mechanically milled for 3 hours and subjected to thermaltreatment at ˜1273K with 5 hours dwell time at the peak temperature. TheXRD patterns of the milled, thermal-treated and copper leached powdershowed the presence of elemental Mo, MoSe₂, and CuSe phase along withthe high intensity peak from (003) plane of graphite at 20 value of26.31° of the 3 hours milled powder. The 3 hours milled powder whenheated to ˜1273K under argon atmosphere showed the formation of fullycrystalline Cu₂Mo₆Se₈+graphite phase along with unreacted MoSe₂.Further, copper was leached out and as a result, the de-cupratedMo₆Se₈+graphite (7:3) phase was obtained. It can be seen from the threeXRD patterns that peak from (003) plane of graphite was present.However, the peak intensity decreased and indicated the collapse ofmicrocrystalline graphite structure and formation of an amorphousdisordered carbon due to defect induced melting commonly observed duringhigh energy mechanical milling.

The electrochemical performance of the in situ Mo₆Se+graphite (7:3) whenevaluated in a 2016 coin cell showed significantly improved performancecompared to Mo₆Se₈ electrode. Although the capacity of in situMo₆Se+graphite (7:3) electrode was lower than Mo₆Se₈ electrode due to70% active mass, it showed an extremely stable capacity up to 100 cycle.The 1^(st) cycle discharge and charge capacity was ˜69.3 mAhg⁻¹ and˜54.8 mAhg⁻¹ with ˜79.1% Coulombic efficiency. Between the 2^(nd) and100 cycle, an average discharge and charge capacity of ˜50.43 mAhg⁻¹ and50.4 mAhg⁻¹ was observed with ˜99.93% Coulombic efficiency. From theabove study, it can be inferred that graphite provided the requiredelectronic path for Mg-ion intercalation/de-intercalation to occurreversibly into and from the Mo₆Se₈ phase and improve the Coulombicefficiency from −96% to ˜99.93%. Capacity versus voltage profile of1^(st), 2^(nd), 3^(rd), 4^(th), 15^(th), and 100^(th) cycle of Mo₆Se₈and Mo₆Se₈+in situ graphite (7:3) showed exactly similar two-stepsmagnesiation at ˜0.96V and ˜1.03V and demagnesiation reaction plateausat ˜1.11V and ˜1.18V was in agreement with cyclic voltammetry data.However, the 1^(st) cycle magnesiation required a slight overvoltage anda single sloping reaction plateau was observed at ˜0.85V andcorrespondingly, the de-magnesiation reaction occurred at ˜1.15V and˜1.20V due to the kinetic barrier that existed in the Chevrel phasehost. Rate retention of Mo₆Se₈ electrode was also good, at C/4, 3 C/8,C/2, 3 C/4, 1 C (1 C˜80 mAg⁻¹), 3 C/2, and 2 C current rate givingaverage discharge capacity of ˜71.3 mAhg⁻¹, ˜60.5 mAhg⁻¹, ˜56.2 mAhg⁻¹,˜51.7 mAhg⁻¹, ˜47.7 mAhg⁻¹, ˜41.9 mAhg⁻¹, and ˜31.2 mAhg⁻¹ was achievedwith Coulombic efficiency ˜96.1%, ˜97.5%, ˜98.7%, ˜99.5%, ˜99.3%,˜99.8%, and ˜99.9%, respectively. From the above study it appears thatHEMM approach is also suitable for rapid synthesis of Cu₂Mo₆Se₈ Chevrelphase, another potential cathode for rechargeable magnesium battery.

Conclusions

High energy mechanical milling is a suitable approach for preparingternary Chevrel phase (Cu₂Mo₆Z₈; Z═S, Se) using metal and metal sulfideas precursors. Cu₂Mo₆S₈ was synthesized from CuS, Mo and MoS₂composition by using the higher energy mechanical milling (HEMM)approach. Mechanical milling of 2 CuS+3Mo+3MoS₂ in the correspondingstoichiometric composition for 30 minutes to 3 hours formed ahomogeneous intricate mixture which upon heating at elevated temperatureformed the desired ternary phase. Quantitative X-rays diffraction studyshowed ˜98% Cu₂Mo₆S₈ phase could be obtained with 30 minutes of millingfollowed by thermal treatment at 1123K for 30 minutes under argon. SEMimages showed the formation of submicron to micrometer sizedagglomerated particles. TG-DSC curves were able to show that elementalMo reacted with CuS and nucleated the Cu_(1.3)Mo₃S₄ phase first whichfurther reacted with MoS₂, and unreacted Mo and formed the desiredCu₂Mo₆S₈ phase. Electrochemical performance of de-cuprated Mo₆S₈ phasein a magnesium battery exhibited competitive performance wheremagnesiation and de-magnesiation was observed at ˜1.0V and ˜1.28Vrespectively. Galvanostatic cycling data showed the 1st cycle dischargeand charge capacity of ˜92 mAhg⁻¹ and ˜57 mAhg⁻¹ with Coulombicefficiency 62%. The Mo₆S₈ electrode was able to deliver a specificcapacity ˜70 mAhg⁻¹ up to 330 cycles with 99% Coulombic efficiency in acoin cell setup demonstrated the structural integrity of theHEMM-derived Mo₆S₈ phase. The corresponding Cu₂Mo₆Se₈ phase was alsosynthesized by thermal treatment of the milled powder at 1151K for 30minutes with ˜96% yield with unreacted Mo, MoSe₂ and MoO₂ as minorimpurities. Electrochemical data of Mo₆Se₈ electrode showed a two-stepmagnesiation (at ˜1.04V and ˜0.96V) and de-magnesiation (at 1.11V and˜1.18V) phenomena. Galvanostatic cycling of Mo₆Se₈ electrode showed a1^(st) cycle discharge and charge capacity of ˜83.5 mAhg⁻¹ and ˜76.3mAhg⁻¹, respectively with ˜91.4% Coulombic efficiency. A stabledischarge and charge capacity of ˜71.2 mAhg⁻¹ and ˜74.2 mAhg⁻¹ wasobserved with 4% fade per cycle likely due to ˜107% stress generated atthe electrode due to change in the crystal structure of magnesiatedMo₆Se₈ from rhombohedral (R3) to triclinic (P1) phase which resulted ina loss in the electrical path of active mass with current collector.Conductive carbon matrix embedded Mo₆Se₈ particles generated via millingapproach were capable of salvaging the fade in capacity and a stablespecific capacity of ˜50 mAhg⁻¹ was observed up to 100 cycles with˜0.07% fade per cycle using graphite and PMAN as the carbon source.

In summary, sulfur and selenium ternary CPs synthesized by the timesaving, scalable HEMM approach, and binary CPs obtained by acid leachingcopper thereafter were shown as cycle stable Mg-ion battery cathodessuitable for electrical energy storage applications.

Example XIV (Cu₁Mo₆S₈)

Experimental

The Cu₂Mo₆S₈ (Cu₂ CP) Chevrel phase was synthesized by using the highenergy mechanical milling (HEMM) route. The approach involved takingstoichiometric amounts of MoS₂, Mo, and CuS batched in a SS vial(powder: ball ratio=1:10). The powders were mechanically milled for 1 h,2 h, and 3 h intervals. After 3 h milling, the powder was heat-treatedat 1000° C. with a heating rate 10° C./min and kept at the peaktemperature for 5 h under ultra-high purity (UHP) Ar atmosphere. XRDpattern of the heat-treated powder showed the formation of purecrystalline Cu₂Mo₆S₈. Further, Cu₂Mo₆S₈ was washed with hydrochloricacid for 2 days to yield a completely crystalline Cu₁Mo₆S₈ (Cu₁ CP)phase. It was known that Mg²⁺ insertion into the Mo₆S₈ Chevrel phaseoccurs in two stages, and therefore offers a capacity ˜120 mAhg⁻¹. Dueto partial charge entrapment after the initial magnesiation reactiononly ˜50-60% magnesium-ion could be extracted resulting ˜40-50%irreversible loss in the 1st cycle from the theoretical value (˜120mAhg⁻¹). In order to minimize the 1^(st) cycle irreversible loss (FIR),Cu ions were partially leached from the original Cu₂ CP structure.Therefore, 1.8 g of Cu₂ CP was added to 20 ml 6 M HCl solution in asmall glass vial with a magnetic stir bar. The Cu₂ CP/HCl solution wascontinuously stirred for 2 days at room temperature. After 2 days ofcontinuous stirring, the solution was ultrasonically cleaned usingdistilled water (3 times) and dried at 60° C. for 24 h. The XRD patternof the partial leached Cu₂ CP shows the formation of completelycrystalline Cu₁Mo₆S₈ (Cu₁ CP). Electrodes were fabricated from thesynthesized Cu₁Mo₆S₈ (Cu₁ CP) and tested in 2016 coin cells using 0.4molar 2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte in the voltage windowof 0.5V-1.5V against pure Mg used as the anode following our publishedwork [3]. Results of the electrochemical cycling are provided below.

Results

FIG. 1 shows the XRD pattern of the heat-treated Cu₂ CP powder obtainedby the precursor route. The Bragg diffraction lines were indexed to ahexagonal-rhombohedral symmetry unit cell of Cu₂Mo₆S₈ (space group: R-3;number: 148; JCPDS-ICDD: 00-047-1519). Lattice parameter(s) calculatedusing the least-square method from the collected experimental data(a=0.96478 nm, c=1.02026 nm, and unit cell volume=822.42×10⁻³ nm³) werein good agreement with the standard Cu₂Mo₆S₈ unit cell parameters(a=0.9584 nm, c=1.025 nm, unit cell volume=815.36×10⁻³ nm³). Similarly,FIG. 1 exhibits the XRD pattern obtained after removal of one copperfrom the heat-treated powder using the hydrochloric acid treatment. TheBraggs lines were indexed with the hexagonal-rhombohedral symmetry unitcell of CuMo₆S₈ phase (space group: R-3; number: 148; JCPDS-ICDD:00-034-1379), and the calculated lattice parameter(s) (a=0.94412 nm,c=1.04761 nm, and unit cell volume=808.70×10⁻³ nm³) matched quite wellwith standard unit cell parameters of CuMo₆S₈ obtained from the aboveICDD database (a=0.94120 nm, c=1.04070 nm, unit cell volume=798.40×10⁻³3) The calculated lattice parameters values are also consistent with thestandard lattice parameter(s) values of pristine Cu₂Mo₆S₈ and acidleached CuMo₆S₈ powder obtained from JCPDS-ICDD database suggesting thatthe 6M HCl treatment for 2 days was successful in leaching out 50%copper from the original Cu₂ CP structure and thus yielding Cu₁Mo₆S₈(Cu₁ CP) (Table 6)

TABLE 6 Calculated lattice parameter(s) of Cu₂Mo₆S₈ and Cu₁Mo₆S₈ alongwith cell parameter(s) with standard ICDD. Unit cell Material a (Å) c(Å) volume (10⁶ × pm³) HEMM derived Cu₂Mo₆S₈ 9.6478 10.2026 822.42Standard Cu₂Mo₆S₈ 9.5840 10.250 815.36 (ICDD: 00-047-1519) HCl leachedCuMo₆S₈ 9.4412 10.4761 808.70 Standard CuMo₆S₈ 9.4120 10.4070 798.40(ICDD: 00-034-1379)

FIG. 2a displays the variation in specific capacity versus cycle numberalong with Coulombic efficiency of the acid leached Cu₁Mo₆S₈ electrode,cycled at a constant current of ˜20 mAg⁻¹ (˜C/6 rate) in the potentialwindow of 0.5-1.5 V using 0.4 molar 2(PhMgCl—AlCl₃)/tetrahydrofuranelectrolyte. As observed in FIG. 2a , the 1^(st) cycle discharge andcharge capacity of the Cu₁Mo₆S₈ electrode is ˜105 mAhg⁻¹ and ˜78 mAhg⁻¹,respectively, with a 1^(st) cycle irreversible loss of ˜25.7% (orCoulombic efficiency of ˜74.3%). However, from the 10^(th) cycle onwardwe can see the electrode maintaining a steady charge-discharge capacityof ˜55 mAhg⁻¹, with a Coulombic efficiency of ˜99.9%. It is noted that afirst cycle irreversible loss of ˜50% is seen for the Mo₆S₈ CP obtainedby completely leaching copper out of HEMM derived Cu₂ CP structures. Thepreliminary results obtained for Cu_(i) CP is therefore encouraging andsuggest that partial Mg²⁺ charge entrapment which is common during the1^(st) cycle magnesiation (discharge) in Mo₆S₈ cathode can be partiallyovercome with Cu₁Mo₆S₈ structure where one Mg²⁺ can cycle without anyhindrance. FIG. 2b shows the galvanostatic charge-discharge profile(1^(st), 2^(nd), 3^(rd), 10^(th), 20^(th), 50^(th) and 90^(th) cycle) ofthe Cu₁Mo₆S₈ electrode conducted at a constant current rate 20 mAg⁻¹(˜C/6). During the 1^(st) cycle, a sloping voltage curve is observed forMg²⁺ insertion/extraction owing to kinetic limitation requiring slightovervoltage of ˜200-300 mV from the equilibrium magnesiation potentialof ˜1.1 V. However, from 10^(th), 20^(th), 50^(th), and 90^(th) cyclesonwards single reaction plateaus are observed at ˜1.1 V (Mg²⁺ insertion)and at ˜1.2 V (Mg²⁺ extraction), respectively. FIG. 2c shows the ratecapabilities of the Cu₁Mo₆S₈ electrode at various current rates of 5mAg⁻¹ (˜C/24), 10 mAg⁻¹ (˜C/12), 20 mAg⁻¹ (˜C/6), 30 mAg⁻¹ (˜C/4), 40mAg⁻¹ (˜C/3), and 60 mAg⁻¹ (˜C/2). The Cu₁Mo₆S₈ delivers averagedischarge capacity of ˜57 mAhg⁻¹, ˜56 mAhg⁻¹, ˜44 mAhg⁻¹, ˜40 mAhg⁻¹,˜32 mAhg⁻¹, ˜29 mAhg⁻¹ at ˜C/24, ˜C/12, ˜C/6, ˜C/4, ˜C/3, and ˜C/2 rateswith Coulombic efficiency ˜97%, ˜100%, ˜98.6%, ˜98.7%, 99.1%, and˜98.7%, respectively.

In order to understand the reaction mechanism, the Cu₁ CP electrode wascycled at a slow current rate of ˜5 mAg⁻¹ (˜C/24 rate) within thepotential window of 0.5-1.5 V using 0.4 molar2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte in a Mg cell. The 1^(st)cycle discharge (magnesiation) and charge (demagnesiation) capacity wasobserved ˜81 and ˜80 mAhg⁻¹ with 1st cycle irreversible loss ˜1.23%implying partial Mg-ion trapping that is common to the Chevrel phaseelectrodes (Mo₆T₈, T═S, Se) can be avoided (see FIG. 3a ). In the 2^(nd)and 3^(rd) cycle ˜99% Coulombic efficiency was observed. Noticeably, thedifferential capacity versus voltage curves (dQ/dV versus V) of the2^(nd) and 3^(rd) cycle illustrates four pairs of redox peaks observedat ˜1.22 V, ˜1.15V, ˜1.1V, and ˜0.8V during discharge and ˜1.04V,˜1.18V, ˜1.23V, and ˜1.32V during charge implying that themagnesiation/de-magnesiation reactions in the synthesized Cu₁ CP occursin various stages (see FIG. 3b ). In order to understand the phaseformation, 2016 coin cells were cycled at a slow current rate of ˜5mAg⁻¹ between ˜0.5V and ˜1.5V cut-off voltage. The cells were thendisassembled inside an UHP Ar filled glove box and washed with anhydroustetrahydrofuran (THF) followed by vacuum drying. Ex-situ XRD analysis ofthe cycled electrodes compared with the bare electrode interestinglyindicate partial copper leaching and simultaneous Mg-ionintercalation/de-intercalation with the formation of Cu_(1-x)Mg_(x) CPand Cu_(1-y)Mg_(y) CP (y<x; x,y<1) at the discharge and charge states,respectively (see FIG. 3c ). In the 2^(nd) cycle, complete copperleaching and exchange with one Mg-ion intercalation occurred during thedischarge cycle with the formation of Mg₁ CP, whereas complete Mg-ionremoval did not occur during the charge cycle leading to the formationof Cu_(1-y)Mg_(y) CP (y<1). In the subsequent cycles however, singleMg-ion intercalation along with partial copper leaching occurred betweenthe two end members of Cu₁ CP and Mg₁ CP resulting in suppression ofMg-ion trapping akin to CP and thereby resulting in improved Coulombicefficiency. More importantly, it should be noted that the cycling dataof Cu₁ CP between 2^(nd)-50^(th) cycle at a slow current rate of ˜5mAg⁻¹ shows an average discharge-charge capacity of ˜55 mAhg⁻¹ yieldinga Coulombic efficiency ˜98% (see FIG. 3d ). The interesting aspect isthe ability of the CuCP to reversibly intercalate one Mg²⁺ ion and alsoreversibly cycle Cu²⁺ ion.

Example XV (Mo₆S_(8-x)Se_(x) 1<x<8)

Experimental: Synthesis of Cu₂Mo₆S₇Se₁/Mo₆S₇Se₁ by the High EnergyMechanical Milling (HEMM) Route

Stoichiometric amounts of MoS₂ (0.875 g), MoSe₂ (0.1426 g), Mo (0.579g), CuS (0.35 g) and CuSe (0.0534 g) were batched in a SS vial (powder:ball ratio=1:10). The powders were mechanically milled for 3 h andsubjected to XRD analysis. After 3 h milling, the powder washeat-treated at 1000° C. for 5 h under UHP Ar atmosphere. XRD pattern ofthe heat-treated powder shows the formation of fully crystalline, pureCu₂Mo₆S₇Se₁. The resultant Cu₂Mo₆S₇Se₁ was then subsequently washed withHCl/O₂ bubbling for 7 h and the XRD pattern collected on the leachedmaterial confirms the formation of Mo₆S₇Se₁ (see FIG. 4). Electrodeswere again fabricated as in the case of Example XIV. Accordingly,electrodes were fabricated from the synthesized Mo₆S₇Se (CP) and testedin 2016 coin cells using 0.4 molar 2(PhMgCl—AlCl₃)/tetrahydrofuranelectrolyte in the voltage window of 0.5V-1.8V against pure Mg used asthe anode following our published work. Results of the electrochemicalcycling are provided below.

Results

The cyclic voltammogram (CV) in FIG. 5a acquired at a sweep rate of ˜50μVs-1 between 0.5-1.8 V versus Mg/Mg²⁺ shows the magnesiation reaction(˜0.98V) and demagnesiation reaction (˜1.3 V, and ˜1.56 V) eventsoccurring in the Mo₆S₇Se electrode respectively, in the very 1^(st)cycle. In the 2^(nd) cycle, 2 pairs of magnesiation and demagnesiationpeaks were observed at ˜1.08 V, ˜1.0 V, ˜1.3 V, ˜1.56 V, respectively.FIG. 5b shows the variation of specific capacity versus cycle numberalong with Coulombic efficiency of the Mo₆S₇Se₁ electrode, cycled at aconstant current of ˜20 mAg⁻¹ (˜C/6 rate) in the potential window of0.5-1.8 V using 0.4 molar 2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte.The galvanostatic cycling result shows a 1^(st) cycle discharge(magnesiation) and charge (demagnesiation) capacity of ˜68.5 mAhg⁻¹(˜58% of the theoretical capacity of ˜115.6 mAhg⁻¹) and ˜66.3 mAh·g⁻¹,respectively, with a Coulombic efficiency of ˜96.8%. Between the 2^(nd)and 200^(th) cycles, an average discharge and charge capacity of ˜66 and64 mAhg⁻¹, respectively, were observed with ˜97% Coulombic efficiency(see FIG. 5b ). The capacity versus voltage profiles of the 2^(nd),100^(th), and 200^(th) cycles shown in FIG. 5c , exhibiting two pairs ofmagnesiation (˜1.17 V and ˜1.13 V) and demagnesiation (˜1.17 V and ˜1.45V) reaction plateaus, respectively, in agreement with the differentialcapacity versus voltage curves (see FIG. 5d ). The results above showthe ability of the sulfo-selenide CP to reversibly cycle Mg²⁺ ions inthe voltage window of 0.5V-1.8V with respect to Mg.

Experimental: Synthesis of Cu₂Mo₆S_(8-x)Se/Mo₆S_(8-x)Se_(x) (x=3, 4, 5)by the HEMM Route

Cu₂Mo₆S₅Se₃/Mo₆Se₃: Stoichiometric amounts of MoS₂ (0.625 g), MoSe₂(0.428 g), Mo (0.537 g), CuS (0.25 g) and CuSe (0.16 g) corresponding tothe nominal composition of Cu₂Mo₆S₅Se₃ were batched in a SS vial(powder: ball ratio=1:10). The powders were mechanically milled for 3 hand subjected to XRD analysis. After 3 h milling, the powder washeat-treated at 1000° C. for 5 h under UHP Ar atmosphere. XRD pattern ofthe heat-treated powder shows the formation of fully crystalline, pureCu₂Mo₆S₅Se₃. The resultant Cu₂Mo₆S₅Se₃ was washed with HCl/O₂ bubblingfor 7 h and the XRD pattern collected on the resultant powder confirmsthe formation of Mo₆S₅Se₃ (see FIG. 6a ).

Cu₂Mo₆S₄Se₄/Mo₆S₄Se₄: Stoichiometric amounts of MoS₂ (0.5 g), MoSe₂(0.57 g), Mo (0.516 g), CuS (0.23 g) and CuSe (0.214 g) corresponding tothe nominal composition of Cu₂Mo₆S₄Se₄ were batched in a stainless steel(SS) vial (powder: ball ratio=1:10). The powders were mechanicallymilled for 3 h and subjected to XRD analysis. After 3 h milling, thepowder was heat-treated at 1000° C. for 5 h under UHP Ar atmosphere. XRDpattern collected on the heat-treated powder shows the formation of purecrystalline Cu₂Mo₆S₄Se₄. The resultant Cu₂Mo₆S₄Se₄ was washed withHCl/O₂ bubbling for 7 h and the XRD collected on the acid treated powderconfirms the formation of Mo₆S₄Se₄ (see FIG. 6b ).

Cu₂Mo₆S₃Se₈/Mo₆S₃Se₈: Stoichiometric amounts of MoS₂ (0.375 g), MoSe₂(0.7125 g), Mo (0.495 g), CuS (0.15 g) and CuSe (0.2675 g) correspondingto the nominal composition of Cu₂Mo₆S₃Se₅ were batched in a SS vial(powder: ball ratio=1:10). The powders were mechanically milled for 3 hand subjected to XRD analysis. After 3 h milling, the powder washeat-treated at 1000° C. for 5 h under UHP Ar atmosphere. XRD patterncollected on the heat-treated powder shows the formation of purecrystalline Cu₂Mo₆S₃Se. The obtained Cu₂Mo₆S₃Se₅ was washed with HCl/O₂bubbling for 7 h and the XRD collected on the acid treated powderconfirms the formation of Mo₆S₃Se (see FIG. 6c ).

As outlined in EXAMPLE XIV and EXAMPLE XV above, electrodes werefabricated from the synthesized Mo₆S_(8-x)Se_(x) (x=3, 4, 5) materials.Accordingly, electrodes were fabricated from the synthesizedMo₆S_(8-x)Se_(x) (x=3, 4, 5) materials and tested in 2016 coin cellsusing 0.4 molar 2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte in thevoltage window of 0.5V-1.5V for Mo₆S_(8-x)Se_(x) (x=3, 4) and 0.5-1.75Vfor Mo₆S_(8-x)Se_(x) (x=5) against pure Mg used as the anode followingour published work. Results of the electrochemical cycling are providedbelow.

Results

FIG. 7a exhibits the variation of specific capacity versus cycle numberalong with Coulombic efficiency of the Mo₆S₅Se₃ electrode, cycled at aconstant current of ˜20 mAg⁻¹ (˜C/6 rate) in the potential window of0.5-1.5 V using 0.4 molar 2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte.As observed in FIG. 7a , the 1^(st) cycle discharge and charge capacityof the Mo₆S₅Se₃ electrode is 74.2 mAhg⁻¹ and 52.5 mAhg⁻¹, respectively,with a 1^(st) cycle irreversible loss of ˜29.3% (or coulombic efficiencyof ˜70.7%). From the 2^(nd) to 90^(th) cycle however, we observed anaverage stable discharge and charge capacity of ˜50.8 mAhg⁻¹ and ˜51.9mAhg⁻¹ with Columbic efficiency ˜97.9%. FIG. 7b shows the galvanostaticcharge-discharge profile (1^(st), 2^(nd), 10^(th), 20^(th), and 50^(th)cycle) of the Mo₆S₅Se₃ electrode obtained at a constant current rate ˜20mAg⁻¹ (˜C/6). During the 1^(st) cycle, a single plateau is observed at˜0.92 V for Mg²⁺ insertion owing to the kinetic limitation in Mo₆S₅Se₃electrode. However, for the 2^(nd), 10^(th), 20^(th), and 50^(th) cycleonwards sloping charge and discharge reaction plateaus are observedcentered on ˜1.1 V/˜1.25 V for both Mg²⁺ insertion/extraction reactions,respectively.

FIG. 8a illustrates the variation in specific capacity versus cyclenumber along with Coulombic efficiency obtained for the Mo₆S₄Se₄electrode, cycled at a constant current of ˜20 mAg⁻¹ (˜C/6 rate) in thepotential window of 0.5-1.5 V using 0.4 molar2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte. As observed in FIG. 8a ,the 1st cycle discharge and charge capacity of the Mo₆S₄Se₄ electrode is˜63 mAhg⁻¹ and ˜43 mAhg⁻¹, respectively, with a 1^(st) cycleirreversible loss of ˜31.7% (or Coulombic efficiency of ˜68.3%). Fromthe 2^(n)d to 100^(th) cycle we observed an average stable discharge andcharge capacity of ˜40.5 mAhg⁻¹ and ˜38.7 mAhg⁻¹ with Columbicefficiency ˜95.6%. FIG. 8b shows the galvanostatic charge-dischargeprofile (1^(st), 2^(nd), 10^(th), and 20^(th) cycle) of Mo₆S₄Se₄electrode obtained at a constant current rate ˜20 mAg⁻¹ (˜C/6). Duringthe 1st cycle, a single plateau is observed at ˜0.86 V for the Mg²⁺insertion owing to the kinetic limitation in the Mo₆S₄Se₄ electrode.However, for the 2^(nd), 10^(th), and 20^(th) cycle onwardscharacteristic sloping reaction plateaus are observed as in the case ofthe Mo₆S₅Se₃ electrode.

FIG. 9a illustrates the variation of specific capacity versus cyclenumber along with Coulombic efficiency of the Mo₆S₃Se₅ electrode, cycledat a constant current of ˜20 mAg⁻¹ (C/6 rate) in the potential window of0.5-1.75 V using 0.4 molar 2(PhMgCl—AlCl₃)/tetrahydrofuran electrolyte.As observed in FIG. 9a , the 1^(st) cycle discharge and charge capacityof the Mo₆S₃Se₅ electrode is ˜78 mAhg⁻¹ and ˜63 mAhg⁻¹, respectively,with a 1^(st) cycle irreversible loss of ˜19.2% (or Coulombic efficiencyof ˜80.8%). From the 2^(nd) to 200^(th) cycle however, we observed anaverage discharge and charge capacity of 60.6 mAhg⁻¹ and ˜63.5 mAhg⁻¹with Columbic efficiency ˜95.6%. FIG. 9b shows the galvanostaticcharge-discharge profile (1^(st), 2^(nd), 10^(th), 20^(th), and 50^(th)cycle) of Mo₆S₃Se₅ electrode obtained at a constant current rate ˜20mAg⁻¹ (˜C/6). During the 1st cycle, a single reaction plateau isobserved at ˜0.74 V for Mg²⁺ insertion owing to the kinetic limitationin Mo₆S₃Se₅ electrode. However, for the 2^(nd), 10^(th), 20^(th), and50^(th) cycles onwards, characteristic sloping reaction plateaus areobserved.

Rate Capability

FIG. 10 shows the rate capabilities of the synthesized Mo₆S₅Se₃ andMo₆S₄Se₄ electrodes at various current rates of 5 mAg⁻¹ (˜C/24), 10mAg⁻¹ (˜C/12), 30 mAg⁻¹ (˜C/4), 40 mAg⁻¹ (C/3), and 60 mAg⁻¹ (˜C/2). TheMo₆S₅Se₃ electrode delivers average discharge capacity of ˜50 mAhg⁻¹,˜42 mAhg⁻¹, ˜26 mAhg⁻¹, ˜23 mAhg⁻¹, ˜19 mAhg⁻¹ at ˜C/24, ˜C/12, ˜C/4,˜C/3, and ˜C/2 rates whereas, the Mo₆S₄Se₄ electrode delivers an averagedischarge capacity of ˜42 mAhg⁻¹, ˜34 mAhg⁻¹, ˜25 mAhg⁻¹, ˜20 mAhg⁻¹, ˜7mAhg⁻¹ at ˜C/24, ˜C/12, ˜C/4, ˜C/3, and ˜C/2 rates, respectively.

In summary, it is important to note that both Cu₂ CP and CuCP combinedwith sulfo-selenide substituted forms of Mo₆S₇Se₁; Mo₆S₅Se₃; Mo₆S₄Se₄;Mo₆S₃Se₅ were successfully synthesized using the high energy mechanicalmilling (HEMM) approach. The resultant materials were then successfullyconverted to the desired Chevrel phase (CP) following heat treatments inultra-high purity (UHP) Ar environment at temperature of 1000° C. for 5h and then with appropriate acid leaching treatments in 6M HCl and amixture of HCl/O₂ gas bubbling to form the partially-cuprated andde-cuprated substituted sulfo-selenide systems. All of the leachedpartially-cuprated/de-cuprated Chevrel phases are indeedelectrochemically active showing stable capacities in the 51 mAhg⁻¹-66mAg⁻¹. Increasing amounts of Se substitution in the CP phase results inthe CP system exhibiting the highest specific capacity of ˜66 mAhg⁻¹.

Whereas particular embodiments of the invention have been describedherein for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details may be made withoutdeparting from the invention as set forth in the appended claims.

What is claimed is:
 1. An electrochemical cell, comprising: analkali-metal-containing anode; a cathode, comprising: a Chevrel-phasematerial of a formula Mo₆Z¹ _(8-y)Z² _(y) derived from a precursormaterial of a formula M_(x)Mo₆Z¹ _(8-y)Z² _(y), wherein M is a metallicelement, ‘x’ is a number from greater than 0 to 4, ‘y’ is a number fromgreater than 0 to less than 8 and each of Z and Z² is a differentchalcogen with or without the presence of oxygen; and an electrolyte. 2.The electrochemical cell of claim 1, wherein the alkali-metal-containinganode comprises magnesium.
 3. The electrochemical cell of claim 1,wherein the metallic element is selected from the group consisting ofLi, Na, Mg, Ca, Sc, Cr, Mn, Fe Co, Ni, Cu, Zn, Sr, Y, Pd, Ag, Cd, In,Sn, Ba, La, Pb, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu andmixtures thereof.
 4. The electrochemical cell of claim 1, wherein eachof the chalcogen Z and Z² is selected from chemical elements in PeriodicTable Group
 16. 5. The electrochemical cell of claim 1, wherein each ofthe chalcogen Z and Z² is selected from the group consisting of sulfur,selenium, tellurium and mixtures of thereof.
 6. The electrochemical cellof claim 1, wherein M is copper, ‘x’ is 2, Z¹ is sulfur and Z² isselenium.
 7. The electrochemical cell of claim 1, wherein the precursormaterial is formed from a mixture of MZ¹, MZ², MoZ₂ ¹, MoZ₂ ² andmolybdenum.
 8. The electrochemical cell of claim 1, wherein theChevrel-phase material is of a formula Mo₆S₈ which is derived from aprecursor material of a formula Cu₂Mo₆Z¹ _(8-y)Z² _(y), and the saidprecursor material is derived from a mixture of ammoniumtertathiomolybdate and anhydrous copper chloride in the presence ofanhydrous N,N-dimethylformamide.
 9. The electrochemical cell of claim 1,wherein the electrolyte comprises amidomagnesium-based magnesium salttransmetallated with aluminum salt.
 10. The electrochemical cell ofclaim 1, wherein the electrolyte comprises a solution of phenylmagnesium chloride-aluminum chloride and amidomagnesium-based magnesiumsalt transmetallated with an aluminum salt electrolyte.
 11. Theelectrochemical cell of claim 1, wherein the electrolyte comprises asolution of 3-bis(trimethylsilyl)aminophenylmagnesium chloride withaluminum chloride in tetrahydrofuran.
 12. The electrochemical cell ofclaim 1, wherein the said electrochemical cell is a rechargeablebattery.
 13. A method of synthesizing a Chevrel-phase cathode material,comprising: combining stoichiometric amounts of copper (II) sulfide,copper (II) selenide, molybdenum, molybdenum disulfide and molybdenumselenide to form a powder mixture; high-energy mechanically milling thepowder mixture; forming a precursor material of a formula Cu₂Mo₆Z¹_(8-y)Z² _(y), with or without the presence of oxygen; and removing acopper element from the precursor material to form a Chevrel-phasecathode material of a formula Mo₆Z¹ _(8-y)Z² _(y) and partially removinga copper element from the precursor material of Cu₂Mo₆S₈ to formCu₁Mo₆S₈.
 14. An electrode, comprising: a slurry, comprising aChevrel-phase cathode material of a formula Mo₆Z¹ _(8-y)Z² _(y) andCu₁Mo₆S₈; and a current collector, wherein the slurry is at leastpartially deposited onto the current collector to form a coatingthereon.